arch: Remove stub cputime.h headers

Many architectures have a stub cputime.h that only include the default
cputime.h

Lets remove the useless headers, we only need to mention that we want
the default headers on the Kbuild files.

Cc: Archs <linux-arch@vger.kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>