powerpc: Drop clang workaround for builtin constant checks

The CPU/MMU feature code has build-time checks that the feature value is
a builtin constant.

Back when the code was added clang wasn't able to compile the
checks, so an ifdef was added to avoid the checks for clang builds.
See commit b5fa0f7f ("powerpc: Fix build failure with clang due
to BUILD_BUG_ON()")

These days clang 13 and later are able to build the checks successfully,
so drop the workaround.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240509121248.270878-1-mpe@ellerman.id.au