selftests/bpf: Fix out-of-srctree build

Building BPF selftests out of srctree fails with:

  make: *** No rule to make target '/linux-build//ima_setup.sh', needed by 'ima_setup.sh'.  Stop.

The culprit is the rule that defines convenient shorthands like
"make test_progs", which builds $(OUTPUT)/test_progs. These shorthands
make sense only for binaries that are built though; scripts that live
in the source tree do not end up in $(OUTPUT).

Therefore drop $(TEST_PROGS) and $(TEST_PROGS_EXTENDED) from the rule.

The issue exists for a while, but it became a problem only after commit
d68ae498 ("selftests/bpf: Install all required files to run selftests"),
which added dependencies on these scripts.

Fixes: 03dcb784 ("selftests/bpf: Add simple per-test targets to Makefile")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230208231211.283606-1-iii@linux.ibm.com