Fix warnings raised by clang 11 (#8664)

This fixes a few kinds of warnings:
 - inconsistent-missing-override when the overriding site is missing the
   override keyword
 - unused-function when a function is neither visible nor used
 - unused-private-field when a private field exists but is never used
 - sign-compare when unsigned ints are compared to signed ints. Not all
   of those have been addressed, but this warning isn't enabled by
   default.

This fixes #8612.

Author: Kernald

src/google/protobuf/compiler/js/js_generator.cc

@@ -94,10 +94,6 @@ bool IsReserved(const std::string& ident) {
   return false;
 }
 
-bool StrEndsWith(StringPiece sp, StringPiece x) {
-  return sp.size() >= x.size() && sp.substr(sp.size() - x.size()) == x;
-}
-
 std::string GetSnakeFilename(const std::string& filename) {
   std::string snake_name = filename;
   ReplaceCharacters(&snake_name, "/", '_');