Enshrine that all ToolchainContexts in a toolchain collection have the same target platform so the toolchain collection should be able to directly access it.

Along the way, rm an unused method.

PiperOrigin-RevId: 333365303
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/BUILD b/src/main/java/com/google/devtools/build/lib/analysis/BUILD
index d4461b2..15c1d3f 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/BUILD
+++ b/src/main/java/com/google/devtools/build/lib/analysis/BUILD
@@ -1056,6 +1056,7 @@
     srcs = ["ToolchainCollection.java"],
     deps = [
         ":toolchain_context",
+        "//src/main/java/com/google/devtools/build/lib/analysis/platform",
         "//src/main/java/com/google/devtools/build/lib/cmdline",
         "//third_party:auto_value",
         "//third_party:guava",