scons: Remove -Werror for the gem5 21.0 release

While gem5 compiles on all our supported compilers, removing the -Werror
flag on the stable branch ensures that, as new compilers are released
with stricter warnings, gem5 remains compilable.

Change-Id: Ic7bb17e770684211330c09143bd8a26720becb9a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/43425
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -322,12 +322,6 @@ if main['GCC'] or main['CLANG']:
     if GetOption('gold_linker'):
         main.Append(LINKFLAGS='-fuse-ld=gold')
 
-    # Treat warnings as errors but white list some warnings that we
-    # want to allow (e.g., deprecation warnings).
-    main.Append(CCFLAGS=['-Werror',
-                         '-Wno-error=deprecated-declarations',
-                         '-Wno-error=deprecated',
-                        ])
 else:
     error('\n'.join((
           "Don't know what compiler options to use for your compiler.",