Revert "perf build: Warn for BPF skeletons if endian mismatches"

This reverts commit 51924ae6

.

We need to better polish building with BPF skels, so revert back to
making it an experimental feature that has to be explicitely enabled
using BUILD_BPF_SKEL=1.

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