Revert "bpf: Allow use of skc_to_mptcp_sock in cg_sockopt"

bpf_core_cast() can be used in mptcp_subflow bpf_iter selftests to get the
msk, instead of using bpf_skc_to_mptcp_sock(). No need to add this patch
anymore, revert it.

This reverts commit d8d42f5a4542c2dc31e4e00c58508a821c6ab789.

Author: Geliang Tang

kernel/bpf/cgroup.c

@@ -2358,8 +2358,6 @@ cg_sockopt_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 #ifdef CONFIG_INET
 	case BPF_FUNC_tcp_sock:
 		return &bpf_tcp_sock_proto;
-	case BPF_FUNC_skc_to_mptcp_sock:
-		return &bpf_skc_to_mptcp_sock_proto;
 #endif
 	case BPF_FUNC_perf_event_output:
 		return &bpf_event_output_data_proto;