bpf: remove unused variable

fix the following warning
../kernel/bpf/syscall.c: In function ‘map_lookup_and_delete_elem’:
../kernel/bpf/syscall.c:1010:22: warning: unused variable ‘ptr’ [-Wunused-variable]
  void *key, *value, *ptr;
                      ^~~

Fixes: bd513cd0

 ("bpf: add MAP_LOOKUP_AND_DELETE_ELEM syscall")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>