Remove the customization of run_in_executor for Android.

[freakboy3742]

The Android event loop integration uses a stub implementation for run_in_executor that effectively disables the ability to run blocking code in a thread. This override doesn't appear to be necessary any more; this PR removes the override, restoring the default executor behavior.

The previous implementation was marked no-cover, so deleting that code increases code coverage. I used the example code from #2479 to verify that the default implementation worked as expected.

Fixes #2479.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

This code was added in beeware/rubicon-java#49, and the comment indicated that it had something to do with DNS. However, the test app in that PR includes DNS lookups, and it works fine on Android with the run_in_executor override removed. So unless @paulproteus can remember what it was for, I think we're OK to merge this.

However, it does raise a couple of questions [EDIT – moved to new issues]:

• Event loop stalls in network test rubicon-objc#445
• Add asyncio networking test to testbed #2507