bpf: Remove unnecessary CAP_MAC_ADMIN check

While well intentioned, checking CAP_MAC_ADMIN for attaching
BPF_MODIFY_RETURN tracing programs to "security_" functions is not
necessary as tracing BPF programs already require CAP_SYS_ADMIN.

Fixes: 6ba43b76

 ("bpf: Attachment verification for BPF_MODIFY_RETURN")
Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200305204955.31123-1-kpsingh@chromium.org