Remove FallThroughError tests

[eernstg]

The following tests should be deleted, because they rely on the existence of FallThroughError, and that class is being removed:

LibTest/core/FallThroughError/toString_A01_t01.dart
LibTest/core/FallThroughError/FallThroughError_A01_t01.dart

[eernstg]

Closing: This was already done in #1563.

[mit-mit]

Hi @sgrekhov, can we remove these from co19_2 too?