Warn about forgotten string interpolators.

In the compiler sources this arrives with a number of false
positives, because we frequently work with strings containing
$foo where foo is an in-scope identifier. I think in normal
source code this will be less of a problem, or none at all;
but to be conservative the warning is born under -Xlint.
Eliminate a bunch of -Xlint warnings.

Mostly unused private code, unused imports, and points where
an extra pair of parentheses is necessary for scalac to have
confidence in our intentions.
Removed dead src directory.

One line of commentary referenced from nowhere can't be very useful.
Merge pull request #2239 from paulp/pr/warnings

Warnings removal and other cleanup.