Kconfig.debug: simplify the dependency of DEBUG_INFO_DWARF4/5

Commit c0a5c81c ("Kconfig.debug: drop GCC 5+ version check for
DWARF5") could have cleaned up the code a bit more.

"CC_IS_CLANG &&" is unneeded. No functional change is intended.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>