mm/oom-kill: Remove obsolete IS_ENABLED check

CONFIG_HAVE_LOW_MEMORY_KILLER is no longer present on 5.4, so remove a
check to see if it's enabled or not.

Change-Id: Ifcd88a0e29b7da6dede7f98fcb4c99ffc7bf3c02
Signed-off-by: Chris Goldsworthy <cgoldswo@codeaurora.org>