Respect user target always

[som-snytt]

Reverts the commit cc60058 in which I seem to have changed my mind whether to respect explicit --target in the presence of explicit --release.

The possibly dubious use case is to work around scala/bug#12761.

Note that if you're going to ignore my compiler option, you should at least tell me so. The deprecation is ambiguous.

warning: -target is deprecated: Use -release instead to compile against the correct platform API.

[som-snytt]

"How could the unit test fail?"

[error] Test scala.tools.nsc.settings.TargetTest.respect user target failed: java.lang.AssertionError: Error output: '11' is not a valid choice for '-release', took 0.002 sec

I forgot to break out the old JDK 8, I think it's still in a box on a shelf in the closet.

[som-snytt]

Maybe consider contributing lately and eightly to testkit.

[som-snytt]

The settings cannot warn (!) as they only have an error function, but it could warn in the backend if user tried to do this:

$ skalac -d /tmp --release:11 --target:8 t7977.scala
warning: target platform version 8 is older than the release version 11
warning: 1 deprecation; re-run with -deprecation for details
2 warnings

[lrytz]

Thanks, this makes sense 👍