cp: Revert "If copy_file_range(2) fails with EXDEV, use fall-back."

This reverts commit 64333654908153cd8e2dcb408c57cc3e3c4f29af.

The error is not valid per api contract, it showed up as a regression
after 15f0b8c309dea1dcb14d3e374686576ff68ac43f

15f0b8c309dea1dc ("zfs: merge openzfs/zfs@9cd71c860 (master)") and was
subsequently in d012836fb6165494 ("zfs: fix up EXDEV handling for
clone_range").

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