bpf: Cleanup unused function declaration

All usage and the definition of `bpf_prog_free_linfo()` has been removed
in commit e16301fb ("bpf: Simplify freeing logic in linfo and
jited_linfo"). Clean up its declaration in the header file.

Signed-off-by: Ruiqi Gong <gongruiqi@huaweicloud.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/all/20230602030842.279262-1-gongruiqi@huaweicloud.com/
Link: https://lore.kernel.org/bpf/20230606021047.170667-1-gongruiqi@huaweicloud.com