Remove mdbx support

[Kerollmops]

This PR removes every reference to the MDBX database, removing its support by the same, and updates the CI to only check the LMDB compilation.

[Kerollmops]

bors merge

[dvic]

May I ask what is the reason for removing mdbx support?

[Kerollmops]

Indeed, I should have explained (I thought I did) but the list of open issues explain that by themselves.

• I was not able to compile MDBX on Windows ad didn't have this time to debug.
• It was making the code base more complex and less Rust idiomatic.
• We were not using MDBX and only LMDB for Meilisearch, nobody helped me fix it so I removed the support of it.

[dvic]

Thanks for clearing that up, makes sense!

[dvic]

By the way, the description of the repo still mentions mdbx :)

[Kerollmops]

Thank you very much, fixed!