libbpf: cleanup Makefile, remove unused elements

The plugin_dir_SQ variable is not used, remove it.
The function update_dir is also unused, remove it.
The variable $VERSION_FILES is empty, remove it.

These all originates from the introduction of the Makefile, and is likely a copy paste
from tools/lib/traceevent/Makefile.

Fixes: 1b76c13e ("bpf tools: Introduce 'bpf' library and add bpf feature check")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>