Fix various warnings (unused, implicits, match)

[kitbellew]

No description provided.

[kitbellew]

@bjaglin couldn't fix all of them (and some are from integrations which could be intentional).

[bjaglin]

Thanks, was your goal to enable -XFatal-warnings to remove warts?

There is a potentially more impactful journey I barely started long time ago with #1744, in order to identify feature gaps as the scalameta trees evolved. I will probably throw an agent at it some time this year.

[bjaglin]

Thanks, was your goal to enable -XFatal-warnings to remove warts?

sorry I missed the Wconf

[kitbellew]

Thanks, was your goal to enable -XFatal-warnings to remove warts?

that would be nice. i had a simpler, purely "egotistical" motivation: while working on the OrganizeImports problem, i would run tests locally and then would have to scroll through lots and lots of warnings, to find the needle (error) in that haystack. so i decided to clean it up a little :)