selftests/bpf: Remove intermediate test files.

The test of linking process creates several intermediate files.
Remove them once the build is over.

This reduces the number of files in selftests/bpf/ directory
from ~4400 to ~2600.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240220231102.49090-1-alexei.starovoitov@gmail.com