Re-enable `ReturnValueIgnored` error prone check

The check was disabled (temporarily) to make progress at the time and never got re-enabled.

Fixes https://github.com/bazelbuild/rules_java/issues/284

PiperOrigin-RevId: 816661241
Change-Id: I8aa0951a7963e5ca4e22a93a4d0723aeb62ead40
diff --git a/toolchains/default_java_toolchain.bzl b/toolchains/default_java_toolchain.bzl
index 4f46393..f836f8f 100644
--- a/toolchains/default_java_toolchain.bzl
+++ b/toolchains/default_java_toolchain.bzl
@@ -59,8 +59,6 @@
     "--should-stop=ifError=FLOW",  # See b/27049950, https://github.com/google/error-prone/issues/4595
     "-g",
     "-parameters",
-    # https://github.com/bazelbuild/bazel/issues/15219
-    "-Xep:ReturnValueIgnored:OFF",
     # https://github.com/bazelbuild/bazel/issues/16996
     "-Xep:IgnoredPureGetter:OFF",
     "-Xep:EmptyTopLevelDeclaration:OFF",