Revert "Implement fallback for sendmmsg and recvmmsg"

This reverts commit e6f1844.

e6f1844 called `sendmmsg` and `recvmmsg` through `libc::syscall` instead of
`libc::sendmmsg` and `libc::recvmmsg`, thus preventing linking issues on old
Android systems where `libc::sendmmsg` and `libc::recvmmsg` isn't available.

In #1503 (comment) the
decision was made to no longer support these old Android systems (API level 16).

This commit reverts e6f1844. Given that `sendmmsg` support was previously
dropped in ee08826, only the `recvmmsg` calls are reverted.

quinn-udp/src/unix.rs

@@ -442,25 +442,9 @@ unsafe fn recvmmsg_with_fallback(
     let flags = 0;
     let timeout = ptr::null_mut::<libc::timespec>();
 
-    #[cfg(not(any(target_os = "freebsd", target_os = "netbsd")))]
-    {
-        let ret =
-            libc::syscall(libc::SYS_recvmmsg, sockfd, msgvec, vlen, flags, timeout) as libc::c_int;
-        if ret != -1 {
-            return ret;
-        }
-    }
-
-    // libc on FreeBSD and NetBSD implement `recvmmsg` as a high-level abstraction over
-    // `recvmsg`, thus `SYS_recvmmsg` constant and direct system call do not exist
-    #[cfg(any(target_os = "freebsd", target_os = "netbsd"))]
-    {
-        #[cfg(target_os = "freebsd")]
-        let vlen = vlen as usize;
-        let ret = libc::recvmmsg(sockfd, msgvec, vlen, flags, timeout) as libc::c_int;
-        if ret != -1 {
-            return ret;
-        }
+    let ret = libc::recvmmsg(sockfd, msgvec, vlen as _, flags, timeout) as libc::c_int;
+    if ret != -1 {
+        return ret;
     }
 
     let e = io::Error::last_os_error();