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.

(cherry picked from commit e55d30768a83f027f2c36e106e0ad5320723709d)

SConstruct

@@ -504,10 +504,6 @@ if selected_platform in platform_list:
             env.Append(CCFLAGS=["-Werror"])
             if methods.using_gcc(env) and version[0] >= 12:  # False positives in our error macros, see GH-58747.
                 env.Append(CCFLAGS=["-Wno-error=return-type"])
-        else:  # Always enable those errors.
-            # False positives in our error macros, see GH-58747.
-            if not (methods.using_gcc(env) and version[0] >= 12):
-                env.Append(CCFLAGS=["-Werror=return-type"])
 
     if hasattr(detect, "get_program_suffix"):
         suffix = "." + detect.get_program_suffix()