bpf: Remove unused bpf_load_pointer

Remove unused bpf_load_pointer function in filter.h. The last user of it has
been removed with 24dea047 ("bpf, x32: remove ld_abs/ld_ind").

Signed-off-by: He Fengqing <hefengqing@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210330024843.3479844-1-hefengqing@huawei.com