bpf: Remove the preceding __ from __bpf_selem_unlink_storage

__bpf_selem_unlink_storage is taking the spin lock and there is
no name collision also. Having the preceding '__' is confusing
when reviewing the later patch.

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20230308065936.1550103-5-martin.lau@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>