Revert "Allow flags and targets to be specified as sets to allow merging common base configs (#1727)"

Causing https://buildkite.com/bazel/google-bazel-presubmit/builds/70654#018a21e2-7326-4cf2-8cdb-1c809f800142
```
Traceback (most recent call last):
  File "bazelci.py", line 4010, in <module>
    sys.exit(main())
  File "bazelci.py", line 3990, in main
    bazel_version=task_config.get("bazel") or configs.get("bazel"),
  File "bazelci.py", line 1382, in execute_commands
    task_config, "test_flags", "test", tmpdir, test_env_vars
  File "bazelci.py", line 1212, in calculate_flags
    flags = list(task_config.get(task_config_key)) or []
TypeError: 'NoneType' object is not iterable
```

This reverts commit 922e89f3f68cab163febbc210366f4edbca2a910.
diff --git a/buildkite/README.md b/buildkite/README.md
index b258f64..948f59c 100644
--- a/buildkite/README.md
+++ b/buildkite/README.md
@@ -190,40 +190,6 @@
     - ":clwb_tests"
 ```
 
-### Sharing Configuration Between Tasks
-
-You can define common configurations and share them between tasks using `*aliases` and the [`<<:` merge key](https://yaml.org/type/merge.html). YAML allows merging maps and sets, but not lists; to merge flags or targets, represent them as a set instead of a list:
-
-```yaml
-# This is a set, not a list, to allow merging into windows.build_flags and windows.test_flags
-.common_flags: !!set &common_flags
-  ? "--incompatible_disable_starlark_host_transitions"
-  ? "--enable_bzlmod"
-
-.common_task_config: &common_task_config
-  build_flags: *common_flags
-  build_targets:
-    - "//..."
-  test_flags: *common_flags
-  test_targets:
-    - "//tests/..."
-
-tasks:
-  ubuntu1804:
-    <<: *common_task_config
-  ubuntu2004:
-    <<: *common_task_config
-  windows:
-    <<: *common_task_config
-    # These are sets, not lists, to allow merging
-    build_flags: !!set
-      <<: *common_flags
-      ? "--noexperimental_repository_cache_hardlinks"
-    test_flags: !!set
-      <<: *common_flags
-      ? "--noexperimental_repository_cache_hardlinks"
-```
-
 ### Specifying a Display Name
 
 Each task may have an optional display name that can include Emojis. This feature is especially useful if you have several tasks that run on the same platform, but use different Bazel binaries.
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 6a52640..0e4fa2d 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -1209,7 +1209,7 @@
             )
         ]
 
-    flags = list(task_config.get(task_config_key)) or []
+    flags = task_config.get(task_config_key) or []
     flags += json_profile_flags
     flags += capture_corrupted_outputs_flags
     # We have to add --test_env flags to `build`, too, otherwise Bazel
@@ -2191,10 +2191,10 @@
 ):
     print_collapsed_group(":dart: Calculating targets")
 
-    build_targets = [] if test_only else list(task_config.get("build_targets", []))
-    test_targets = [] if build_only else list(task_config.get("test_targets", []))
-    coverage_targets = [] if (build_only or test_only) else list(task_config.get("coverage_targets", []))
-    index_targets = [] if (build_only or test_only) else list(task_config.get("index_targets", []))
+    build_targets = [] if test_only else task_config.get("build_targets", [])
+    test_targets = [] if build_only else task_config.get("test_targets", [])
+    coverage_targets = [] if (build_only or test_only) else task_config.get("coverage_targets", [])
+    index_targets = [] if (build_only or test_only) else task_config.get("index_targets", [])
 
     index_targets_query = (
         None if (build_only or test_only) else task_config.get("index_targets_query", None)