Fix #2437: Drop erroneous case in Pickler

[odersky]

The case was a left-over from when we did not pickle
types as trees. There was a special case to record the symbol
of a bound type variable. This is no longer needed as we now
pickle the full tree that defines the symbol.

[smarter]

The testcase from the issue is missing, otherwise LGTM, thanks!

[smarter]

Bintray is having issue right now which mean our CI is useless unfortunately: https://bintray.statuspage.io/ " Investigating - We are experiencing intermittent download issues. We are currently investigating and will update as soon as the issue is resolved. "

[smarter]

One test failed (tests/patmat/exhausting.scala), here's a minimization:

class Foo[T]
object Test {
  def foo[T](arg: Foo[T]) = arg match {
    case bla: Foo[_] =>
  }
}

If you replace Foo[_] by Foo[t] it doesn't crash. Stack trace:

Exception in thread "main" java.lang.AssertionError: assertion failed: unresolved symbols: type _ when pickling try/i2437b.scala
        at scala.Predef$.assert(Predef.scala:170)
        at dotty.tools.dotc.core.tasty.TreePickler.pickle(TreePickler.scala:587)
        at dotty.tools.dotc.transform.Pickler$$anonfun$run$2$$anonfun$apply$1.apply(Pickler.scala:52)
        at dotty.tools.dotc.transform.Pickler$$anonfun$run$2$$anonfun$apply$1.apply(Pickler.scala:45)
        at scala.collection.immutable.List.foreach(List.scala:392)
        at dotty.tools.dotc.transform.Pickler$$anonfun$run$2.apply(Pickler.scala:45)
        at dotty.tools.dotc.transform.Pickler$$anonfun$run$2.apply(Pickler.scala:44)
        at scala.collection.immutable.List.foreach(List.scala:392)
        at dotty.tools.dotc.transform.Pickler.run(Pickler.scala:44)
        at dotty.tools.dotc.core.Phases$Phase$$anonfun$runOn$1.apply(Phases.scala:283)
        at dotty.tools.dotc.core.Phases$Phase$$anonfun$runOn$1.apply(Phases.scala:281)
        at scala.collection.immutable.List.map(List.scala:284)
        at dotty.tools.dotc.core.Phases$Phase$class.runOn(Phases.scala:281)
        at dotty.tools.dotc.transform.Pickler.runOn(Pickler.scala:76)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1$$anonfun$apply$mcV$sp$1.apply(Run.scala:82)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1$$anonfun$apply$mcV$sp$1.apply(Run.scala:79)
        at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
        at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:186)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply$mcV$sp(Run.scala:79)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply(Run.scala:67)
        at dotty.tools.dotc.Run$$anonfun$compileUnits$1.apply(Run.scala:67)
        at dotty.tools.dotc.util.Stats$.monitorHeartBeat(Stats.scala:76)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:67)
        at dotty.tools.dotc.Run.compileSources(Run.scala:64)
        at dotty.tools.dotc.Run.compile(Run.scala:48)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:26)
        at dotty.tools.dotc.Driver.process(Driver.scala:124)
        at dotty.tools.dotc.Driver.process(Driver.scala:93)
        at dotty.tools.dotc.Driver.process(Driver.scala:105)
        at dotty.tools.dotc.Driver.main(Driver.scala:132)
        at dotty.tools.dotc.Main.main(Main.scala)

[smarter]

This is now missing testcases.

[smarter]

Rebased and added testcases.