Remove the --prune_cpp_modules command line option.

Bazel builds generally don't use modules, so this was a no-op and it's mysterious enough that I'm reasonably confident that no one used it in Bazel (and a Google search agrees)

RELNOTES[INC]: The --prune_cpp_modules command line option is not supported anymore.

PiperOrigin-RevId: 217134320

src/main/java/com/google/devtools/build/lib/rules/cpp/CppCompileActionBuilder.java

@@ -412,7 +412,7 @@ private boolean useHeaderModules() {
   }
 
   private boolean shouldPruneModules() {
-    return cppConfiguration.getPruneCppModules() && shouldScanIncludes && useHeaderModules();
+    return shouldScanIncludes && useHeaderModules();
   }
 
   private void verifyActionIncludePaths(CppCompileAction action, Consumer<String> errorReporter) {

src/main/java/com/google/devtools/build/lib/rules/cpp/CppConfiguration.java

@@ -922,10 +922,6 @@ public boolean getInmemoryDotdFiles() {
     return cppOptions.inmemoryDotdFiles;
   }
 
-  public boolean getPruneCppModules() {
-    return cppOptions.pruneCppModules;
-  }
-
   public boolean getPruneCppInputDiscovery() {
     return cppOptions.pruneCppInputDiscovery;
   }

src/main/java/com/google/devtools/build/lib/rules/cpp/CppOptions.java

@@ -646,19 +646,6 @@ public Label getFdoPrefetchHintsLabel() {
   )
   public boolean inmemoryDotdFiles;
 
-  @Option(
-    name = "prune_cpp_modules",
-    defaultValue = "true",
-    documentationCategory = OptionDocumentationCategory.BUILD_TIME_OPTIMIZATION,
-    effectTags = {
-      OptionEffectTag.LOADING_AND_ANALYSIS,
-      OptionEffectTag.EXECUTION,
-      OptionEffectTag.CHANGES_INPUTS
-    },
-    help = "If enabled, use the results of input discovery to reduce the number of used modules."
-  )
-  public boolean pruneCppModules;
-
   @Option(
     name = "experimental_prune_cpp_input_discovery",
     defaultValue = "false",