Remove duplicates from clang_devteam_warnings

Since the clang_devteam_warnings are appended to the gcc_devteam_warnings
when strict-warnings are requested, any items present in both the gcc
and clang variables will be duplicated in the cflags used for clang builds.
Remove the extra copy from the clang-specific flags in favor of the
gcc_devteam_warnings that are used for all strict-warnings builds.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3239)