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>