selftests/bpf: Remove empty TEST_CUSTOM_PROGS

Commit f04a32b2 ("selftests/bpf: Do not use sign-file as testcase")
removed the TEST_CUSTOM_PROGS assignment, and removed it from being used
on TEST_GEN_FILES. Remove two leftovers from that cleanup. Found by
inspection.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexey Gladkov <legion@kernel.org>
Link: https://lore.kernel.org/bpf/20240216-bpf-selftests-custom-progs-v1-1-f7cf281a1fda@suse.com