Revert "libbpf: allow to use packaged version"

This reverts commit 82749180b23d3c9c060108bc290ae26507fc324e.

Getting in the way of releasing 1.20, breaking the build of a dwarves
rpm when a libbpf package is installed in a fedora 33 system:

In file included from /home/acme/rpmbuild/BUILD/dwarves-1.20/strings.c:7:
/home/acme/rpmbuild/BUILD/dwarves-1.20/pahole_strings.h:9:10: fatal error: bpf/btf.h: No such file or directory
    9 | #include <bpf/btf.h>
      |          ^~~~~~~~~~~

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
9 files changed