Remove unused imports.

[olafurpg]

I've been delaying this commit for too long hoping to fix it with
scalafix. However, it turns out that we can't yet use scalahost in the
scalafix repo because of scalameta/scalameta#979.

I could not get myself to manually remove the imports so I implemented
instead an sbt plugin called sbt-messagehost that collects reported messages
from the compilerReporter in sbt and writes .semanticdb files. Using
messagehost, I was able to run RemoveUnusedImports on the scalafix
codebase. I suspect messagehost may become popular as a lightweight
alternative to scalahost.

I plan to publish messagehost later, it will not be part of the scalafix
repo.

[ShaneDelmore]

This is a great idea. I have been wanting to automate removing unused imports as they are a little annoying to have to keep removing manually but not wanting the full weight of semantic db for a case where the compiler message clearly has the needed information. Love this ❤️

[olafurpg]

Glad you like the idea @ShaneDelmore I needed to make minor accommodations in RemoveUnusedImport to work with sbt-messagehost's offset positions instead of range positions, but it was not too tricky.