Remove automatic version detection

[mekeor]

Lean4-Mode has the feature to automatically detect the version and launch either lean(3)-mode or Lean4-Mode as needed.

Since both Lean version 3 as well as Lean(3)-Mode is deprecated, the benefit of this feature is small. Since the implementation of this feature is quite complex and extensive, it's not adequate to the burden on maintenance work.

As discussed on Zulip#Emacs > `lean4-autodetect-lean3` and cleanup, let's drop this feature.