bpf: Remove unnecessary cpu == 0 check in memalloc

After merging the patch set [1] to reduce memory usage
for bpf_global_percpu_ma, Alexei found a redundant check (cpu == 0)
in function bpf_mem_alloc_percpu_unit_init() ([2]).
Indeed, the check is unnecessary since c->unit_size will
be all NULL or all non-NULL for all cpus before
for_each_possible_cpu() loop.
Removing the check makes code less confusing.

  [1] https://lore.kernel.org/all/20231222031729.1287957-1-yonghong.song@linux.dev/
  [2] https://lore.kernel.org/all/20231222031745.1289082-1-yonghong.song@linux.dev/

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240104165744.702239-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>