Revert "Implement fallback for `sendmmsg` and `recvmmsg`"

[mxinden]

This reverts commit e6f184489e703bcf70850f99373c88746c250662.

e6f18448 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 https://github.com/quinn-rs/quinn/issues/1503#issuecomment-2285962727 the suggestion was made to no longer support these old Android systems (API level 16).

This commit reverts e6f18448. Given that sendmmsg support was previously dropped in ee088265, only the recvmmsg calls are reverted.

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Alternative to https://github.com/quinn-rs/quinn/pull/1964. Given the additional simplicity, my preference is for this change.