luajit: drop MoonJIT package
MoonJIT is no longer active [1]. The repository is read-only, so let's drop this and use LuaJIT from the OpenWrt packages repository.
[1] https://twitter.com/siddhesh_p/status/1308594269502885889?s=20
I tested this on Turris Omnia, HBK branch, and it was smooth to transition.