Revert "perf build: Make BUILD_BPF_SKEL default, rename to NO_BPF_SKEL"

This reverts commit a980755b

.

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>