defconfig: Remove the sm8150[-perf]_defconfig symbolic links

The defconfigs are moved to arch/arm64/configs/vendor/. The
symbolic links were being carried temporarily until all the
build system changes are in place. Hence, removing them as
they are no longer required.

Change-Id: I27bdb15b37679fc4451df90a8101a23148e8efb9
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>