arm64: kernel: Fix mismerge in apply_ssbd

arm64_update_smccc_conduit was open coded in CAF's cherry-pick f045269b
("arm64: Call ARCH_WORKAROUND_2 on transitions between EL0 and EL1"). It
was introduced later in cherry-pick 59f35c70 ("arm64: entry: Add vectors
that have the bhb mitigation sequences") but without removing the customised variant.

Change-Id: Id11cccd88b749c46aaa3f9cd254839d833c97f7d
Signed-off-by: Alexander Winkowski <dereference23@outlook.com>