system_dlkm: Remove manual module copy

Rather than copying GKI modules from source folder
pointed by BOARD_SYSTEM_DLKM_SRC; use core build
macro BOARD_SYSTEM_KERNEL_MODULES as list
of GKI modules to be copied on the system_dlkm partition.

This leverages core infrastructre already present for
the kernel modules for vendor_dlkm, odm_dlkm, etc.

Bug: 232430739
Test: TH
Change-Id: Ic53f450eb426dc4ec38fc80b7ceaa625c8ab206b
Signed-off-by: Ramji Jiyani <ramjiyani@google.com>