Remove system_dlkm from _FRAMEWORK_PARTITIONS

system_dlkm partition is related to GKI, so it should be
in the same category as boot image.

Test: merge_target_files \
        --path otatools \
        --framework-target-files framework-target_files.zip \
	--vendor-target-files vendor-target_files.zip \
	--output-target-files merged-target_files.zip
Bug: 294482075
Change-Id: Ie8f60d72f44d31e1bc3b556735b92ed0a2fda662