padata: Remove unused padata_remove_cpu

The function padata_remove_cpu was supposed to have been removed
along with padata_add_cpu but somehow it remained behind.  Let's
kill it now as it doesn't even have a prototype anymore.

Fixes: 815613da ("kernel/padata.c: removed unused code")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>