xwayland: Remove text/x-moz-url and _NETSCAPE_URL mime converters

The xwayland data bridge tries to be helpful and convert some mimes.
However, that mime conversion code is buggy, and it appears like
Thunderbird can send text/x-moz-url in format, which our bridge doesn't
handle properly.

However, mime type conversion is completely out of the scope of the
compositor. We also can't keep up with various mime types. Given that
X11 clients already must handle _NETSCAPE_URL and text/x-moz-url, this
change removes our mime type conversion helpers. For the record, neither
wlroots-based compositors nor mutter perform such conversion either.

With this change, kwin will send text/x-moz-url and _NETSCAPE_URL data
as is.

BUG: 458226


(cherry picked from commit 498bde9c)