[gardening] Remove lib{,_2}/mirrors/typedef_test

This test has flaked on our CI but actually it's consistently failing
because we do not represent `typedef`s in a reflective way in the VM
since switching to kernel (because they used to be desguared on kernel
level).

So this has not been working for a long time, let's remove this test.

Issue #44489

Change-Id: I36e8d8aee7a7736e8afc5741b01d10429ac0d7a9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/331941
Commit-Queue: Martin Kustermann <[email protected]>
Reviewed-by: Slava Egorov <[email protected]>

Author: mkustermann