TST: Remove explicit connectivity checks in test cases.

Instead, network decorator in pandas.util.testing checks for that instead.
You have to opt into failing on tests by setting
`pandas.util.testing._FORCE_NETWORK_ERROR` to `True`.

Fixes #3910.

This version of the fix keeps the explicit check that you can connect to
`http://www.google.com` as a stand-in for network connectivity.