SCons: Don't enable any `-Werror=return-type` outside `werror=yes`

We shouldn't presume that future compilers will not have false positives or
find new occurrences of this warning, which would break compiling old versions
of the engine without passing custom `CXXFLAGS`.

Follow-up to #60652.

SConstruct

@@ -622,10 +622,6 @@ if selected_platform in platform_list:
                     env.Append(CCFLAGS=["-Wno-error=return-type"])
             elif methods.using_clang(env) or methods.using_emcc(env):
                 env.Append(CXXFLAGS=["-Wno-error=#warnings"])
-        else:  # Always enable those errors.
-            # False positives in our error macros, see GH-58747.
-            if not (methods.using_gcc(env) and cc_version_major >= 12):
-                env.Append(CCFLAGS=["-Werror=return-type"])
 
     if hasattr(detect, "get_program_suffix"):
         suffix = "." + detect.get_program_suffix()