Remove long outdated --experimental_build_setting_api.

This has been true for years and has no actual semantic impact. This is leftover
from the flag-gated introduction of Bazel's Starlark flag API.

PiperOrigin-RevId: 564746589
Change-Id: I4aeea41a997f821f7985ec4ef216aaea168eca61

Author: gregestren

src/main/java/com/google/devtools/build/lib/packages/semantics/BuildLanguageOptions.java

@@ -94,16 +94,6 @@ public final class BuildLanguageOptions extends OptionsBase {
       help = "No-op.")
   public boolean incompatibleDisallowSymlinkFileToDir;
 
-  @Option(
-      name = "experimental_build_setting_api",
-      defaultValue = "true",
-      documentationCategory = OptionDocumentationCategory.UNDOCUMENTED,
-      effectTags = OptionEffectTag.BUILD_FILE_SEMANTICS,
-      help =
-          "If set to true, allows access to value of build setting rules via "
-              + "ctx.build_setting_value.")
-  public boolean experimentalBuildSettingApi;
-
   // TODO(#11437): Delete the special empty string value so that it's on unconditionally.
   @Option(
       name = "experimental_builtins_bzl_path",

src/main/java/com/google/devtools/build/lib/starlarkbuildapi/StarlarkRuleContextApi.java

@@ -204,10 +204,10 @@ public interface StarlarkRuleContextApi<ConstraintValueT extends ConstraintValue
       name = "build_setting_value",
       structField = true,
       doc =
-          "<b>Experimental. This field is experimental and subject to change at any time. Do not "
-              + "depend on it.</b> <p>The value of the build setting that is represented "
-              + "by the current target. It is an error to access this field for rules that do not "
-              + "set the <code>build_setting</code> attribute in their rule definition.")
+          "Value of the build setting represented by the current target. If this isn't the context"
+              + " for an instance of a rule that sets the <a href=\""
+              + "https://bazel.build/extending/config#rule-parameter\"><code>build_setting</code>"
+              + "</a> attribute, reading this is an error.")
   Object getBuildSettingValue() throws EvalException;
 
   @StarlarkMethod(