Remove INTERNAL flag from TASTy

[bishabosha]

There appears to be no reason why there is an INTERNAL flag in TastyFormat, it is never pickled, and when unpickled there is hardwired a crash, this line was updated 6 years ago:

https://github.com/lampepfl/dotty/blob/8957e51230b843a7b3cff6f8e0698aad6f1d778e/compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala#L637

In TastyFormat it is described as meaning "package private (not yet used)", is there still a plan to reserve this as a flag?

cc/ @odersky