bpf: Remove unused variables.

Remove unused prev_offset, min_size, krec_size variables.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202309190634.fL17FWoT-lkp@intel.com/


Fixes: aaa619eb ("bpf: Refactor check_btf_func and split into two phases")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>