Remove useless wrappers of asm-generic/cpumask.h

frv and xtensa both have a header (in their include/asm directories)
that are thin wrappers around asm-generic/cpumask.h. These wrappers are
useless, since that header doesn't exist. They are also unused (all
files including asm/cpumask.h are x86 specific).

hexagon and openrisc generate similar headers at build time (using a
generic-y entry in include/asm/Kbuild). These generated headers are
useless and unused too.

Remove these headers and generic-y entries.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Richard Kuo <rkuo@codeaurora.org>
Acked-by: David Howells <dhowells@redhat.com> [FRV]
Signed-off-by: Michal Marek <mmarek@suse.cz>