Revert "wayland-keyboard: Don't send pressed keys on enter"

Back in 2014 sending pressed keys to Wayland clients caused issues, because at least Xwayland didn't handle that gracefully, causing issues like ghost-pressed keys. A way it was reproduced was quickly alt-tab:ing to and from a Firefox window, which would cause the File menu bar incorrectly appearing.

While this was reported to the Xwayland component back then, it was, probably by mistake, assumed to be an issue in mutter, and mutter stopped sending pressed key events on enter.

The following year, Xwayland was eventually fixed, but the work around in mutter has been kept around until it was again noticed as an inconsistency between compositor implementations.

Lets remove the work around, and follow the spec, again.

This reverts commit c39f18c2.

Closes: #2457 (closed)

This also includes a fix to not double-disable input devices classes when freeing the seat.