cmd/compile/internal/types2: use go/types/testdata/fixedbugs tests

cmd/compile/internal/types2: use go/types/testdata/fixedbugs tests

Since the fixedbugs tests are now identical between the two type checkers,
remove the local copy of the fixedbugs tests and (for now) use the tests
in go/types/testdata/fixedbugs instead. Eventually we may decide to move
all tests out of the type checker directories and place them in a
shared space (e.g. internal/types/testdata).

For #54511.

Change-Id: I451c20c784710c36fa50b1d24ac97af554c572af
Reviewed-on: https://go-review.googlesource.com/c/go/+/426658
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>