overzealous assert in GenBCode

The assert in question was aimed at ruling out
gotos (ie "jumping-applys") in actual argument position
of a jumping-apply.

But the assert in question went overboard
to also rule out a LabelDef in actual argument position.

This commit removes the assert in question altogether.

The unwanted behaviors, and only those, are rule out by
the test added in this commit and the existing tests for SI-6089.

See also https://issues.scala-lang.org/browse/SI-7749
removing defensive code made obsolete by existing fix to SI-5604

SI-5604 was fixed in https://github.com/magarciaEPFL/scala/commit/e6f10b0