Revert "[Modules] Process include files changes (llvm#90319)"

This reverts commit 9a9cff1, which
combined with CAS (where every file has a 0 mtime), causes content
validation to run on every input file.

(cherry picked from commit ffe014a)

Author: bnbarham

clang/lib/Serialization/ASTReader.cpp

@@ -2862,14 +2862,6 @@ InputFile ASTReader::getInputFile(ModuleFile &F, unsigned ID, bool Complain) {
       F.StandardCXXModule && FileChange.Kind == Change::None)
     FileChange = HasInputContentChanged(FileChange);
 
-  // When we have StoredTime equal to zero and ValidateASTInputFilesContent,
-  // it is better to check the content of the input files because we cannot rely
-  // on the file modification time, which will be the same (zero) for these
-  // files.
-  if (!StoredTime && ValidateASTInputFilesContent &&
-      FileChange.Kind == Change::None)
-    FileChange = HasInputContentChanged(FileChange);
-
   // For an overridden file, there is nothing to validate.
   if (!Overridden && FileChange.Kind != Change::None) {
     if (Complain) {