Modernize and simplify Meson files

Use f-strings and some better loops.

Remove `TALKATU_UNUSED` which duplicated `G_GNUC_UNUSED`, and is also unused.

Also remove checks for old Meson which I thought I looked for before, but didn't find.

Testing Done:
Compiled and ran `ninja test`

Reviewed at https://reviews.imfreedom.org/r/2038/