bpf: Remove unused field "mod" in struct bpf_trampoline

It seems that the field "mod" in struct bpf_trampoline is not used
anywhere after the commit 31bf1dbc ("bpf: Fix attaching
fentry/fexit/fmod_ret/lsm to modules"). So we can just remove it now.

Fixes: 31bf1dbc ("bpf: Fix attaching fentry/fexit/fmod_ret/lsm to modules")
Signed-off-by: Menglong Dong <dongmenglong.8@bytedance.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20240128055443.413291-1-dongmenglong.8@bytedance.com