profiling: remove stale percpu flip buffer variables

For some reason I didn't see this issue on my arm64 or x86-64 builds,
but Stephen Rothwell reports that commit 2accfdb7 ("profiling:
attempt to remove per-cpu profile flip buffer") left these static
variables around, and the powerpc build is unhappy about them:

  kernel/profile.c:52:28: warning: 'cpu_profile_flip' defined but not used [-Wunused-variable]
     52 | static DEFINE_PER_CPU(int, cpu_profile_flip);
        |                            ^~~~~~~~~~~~~~~~
  ..

So remove these stale left-over remnants too.

Fixes: 2accfdb7

 ("profiling: attempt to remove per-cpu profile flip buffer")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>