Kernel: Allow Socket subclasses to fail construction

For example, socket(AF_INET) should only succeed for valid SOCK_TYPEs.

Kernel/Net/Socket.cpp

@@ -37,7 +37,6 @@
 
 KResultOr<NonnullRefPtr<Socket>> Socket::create(int domain, int type, int protocol)
 {
-    (void)protocol;
     switch (domain) {
     case AF_LOCAL:
         return LocalSocket::create(type & SOCK_TYPE_MASK);