arm64: remove redundant object file list

Commit 4b3dc967 ("arm64: force CONFIG_SMP=y and remove redundant
#ifdefs") forces SMP on arm64. To build the necessary objects for SMP,
they were added to the arm64-obj-y rule in arch/arm64/kernel/Makefile,
without removing the arm64-obj-$(CONFIG_SMP) rule.

Remove redundant object file list depending on always-yes CONFIG_SMP in
arch/arm64/kernel/Makefile.

Signed-off-by: Jonas Rabenstein <jonas.rabenstein@studium.uni-erlangen.de>
Signed-off-by: Will Deacon <will.deacon@arm.com>