bpf, arm64: Remove unused-but-set function and variable.

Remove unused-but-set function and variable to fix the build warning:
   arch/arm64/net/bpf_jit_comp.c: In function 'arch_bpf_trampoline_size':
    2547 |  int nregs, ret;
         |      ^~~~~

Fixes: 9014cf56 ("bpf, arm64: Support up to 12 function arguments")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Xu Kuohai <xukuohai@huawei.com>
Link: https://lore.kernel.org/bpf/20250528002704.21197-1-alexei.starovoitov@gmail.com

Closes: https://lore.kernel.org/oe-kbuild-all/202505280643.h0qYcSCM-lkp@intel.com/