s390/ftrace: remove dead code

[ Upstream commit b860b934 ]

ftrace_shared_hotpatch_trampoline() never returns NULL,
therefore quite a bit of code can be removed.

Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Stable-dep-of: 2a8db5ec ("RISC-V: Don't check text_mutex during stop_machine")
Signed-off-by: Sasha Levin <sashal@kernel.org>