remove _all_ unused imports

[shmish111]

This is something I use really regularly in purescript-ide, it's a big pain having to go through each import line one-by-one to remove the unused imports.

[pepeiborra]

A code action "Remove all redundant imports" would be a nice way to implement this

[jneira]

This was accomplished by haskell/ghcide#867, already included in 0.6.0