Draft: Build-Depend on unicode-cldr-core and drop Unicode missing-source

@smcv Although not directly used by the gtk4 build, unicode-cldr-core is approximately the source data for emojibase.

Technically emojibase is using cldr-json which is not yet packaged in Debian.

Then, drop the copy of the Unicode data from debian/missing-sources/

We could optionally add a Build-Depends: unicode-data (>= 15.1) but I am leaning towards not.

• https://github.com/milesj/emojibase/commit/f6b2dc94
• https://github.com/unicode-org/cldr-json/tree/main/cldr-json/cldr-annotations-full
• https://bugs.debian.org/966499