Drop libquvi support

No development activity (or even any sign of life) for almost a year.

A replacement based on youtube-dl will probably be provided before the
next mpv release. Ask on the IRC channel if you want to test.

Simplify the Lua check too: libquvi linking against a different Lua
version than mpv was a frequent issue, but with libquvi gone, no
direct dependency uses Lua, and such a clash is rather unlikely.