[Feature] Remove `mode-line-message` on finish

[FrancisMurillo]

Hey dood,

The mode-line-message clutters the mode line in my use case once it is done. I've used langtool-finish-hook to remove the entry `(t langtool-mode-line-message). I think this falls to the library to provide a means or option for this but this is just a minor request.

[mhayashi1120]

Thanks for the request.