Drop BaseRequest.send() method

[natebosch]

This method has very little usage, and it feels unnecessary to have yet another utility that creates a single-request client. If the bit of code which handles creating a StreamedResponse is the useful part we should expose it some other way.

cc @jakemac53 @kevmoo - for a second opinion

[jakemac53]

This does also seem easy to use incorrectly, how often is it that you would only ever send a single request? It seems like it has very little real world value, and is primarily a foot-gun, so SGTM.