Remove custom sphinx httpdomain extension once upstream is fixed

[phlax]

proposed upstream fix is here sphinx-contrib/httpdomain#59

[stevepiercy]

This was released in v1.8.1.

[phlax]

thanks for headsup - we need to remove it from here i think - https://github.com/envoyproxy/toolshed/blob/main/py/envoy.docs.sphinx_runner/envoy/docs/sphinx_runner/ext/httpdomain.py

[stevepiercy]

FYI, I expect a 2.0.0 release this week.