powerpc/signal: Remove BUG_ON() in handler_signal functions

There is already the same BUG_ON() check in do_signal() which
is the only caller of handle_rt_signal64() handle_rt_signal32() and
handle_signal32().

Remove those three redundant BUG_ON().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/3582e10a341d523c9c3f1ac925c3aaefc9d9293d.1597770847.git.christophe.leroy@csgroup.eu