bpf: Remove obsolete WARN_ON call

The WARN_ON call in bpf_trampoline_update could never hit, because we
direct the code path with (total == 0) to out label, which effectively
skips the WARN_ON call.

The WARN_ON made sense back then when it checked tr->selector, but now
with total being set just inside the function it's useless.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20260424153905.354922-2-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>