delete typeshed_experimental

Summary:
this was for an old experiment, and essentially is typeshed_internal with scipy/pandas/arrow stubs swapped out.

originally it was meant for codenav only, but I think some people inadvertently started type checking with it.

the duplication causes confusion, so let's remove it.

Reviewed By: stroxler

Differential Revision: D114153484

fbshipit-source-id: b3694e2e764257304735e2f8ac128ad84707fe7e

Author: yangdanny97

.github/workflows/build-and-test.yml

@@ -114,7 +114,7 @@ jobs:
 
   pyre:
     # NOTE: .pyre_configuration.external has "ignore_all_errors" for tests/ - this is intentional.
-    # Meta's internal Pyre uses python/typeshed_experimental with permissive pandas stubs
+    # Meta's internal Pyre uses python/typeshed_internal with permissive pandas stubs
     # (Axes = ... | list | tuple), but external pyre-check-nightly uses stricter typeshed
     # stubs that reject tuple/list for DataFrame index/columns. This causes ~85 false-positive
     # errors in tests that don't occur internally. The main library code (balance/) is still