Refactor: based: Drop dead code in prepare_input()

Since dropping legacy mode, the only thing that sets PCMKXA_CIB_UPDATE
to true is sync_our_cib(). In that case, the request is
PCMKCIB_REQUEST_REPLACE, which maps to cibop_replace -- not
cibop_apply_patch.

Also drop PCMK__XE_CIB_UPDATE_DIFF, since this was the only occurrence.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>