MFH: r520357 www/py-httmock: Switch to USE_GITHUB, Remove BROKEN

While we prefer using CHEESESHOP when souces are provided there, in the
event it's missing valuable files such as tests, test data, LICENCE, it
is preferable and more robust to use an alternative source entirely, than a
combination of CHEESESHOP (or anything else) combined with another source
for specific files, which can cause issues [1]

This change switches to USE_GITHUB and removes unecessary and associated
Makefile entries accordingly.

[1] ports r516897

See Also:

https://github.com/patrys/httmock/pull/59

PR: 242694
Submitted by: Derek Schrock <dereks lifeofadishwasher com>

Approved by: ports-secteam (blanket: fetch/build fix)