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: https://gitlab.gnome.org/GNOME/mutter/-/issues/2457
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2657>