Remove pip-wheel-metadata/ from Python.gitignore (#3364)

pip generated this folder for a few versions, as part of it's initial
implementation of PEP 517.

pip has not generated this folder for a few versions now, so it should
be OK to remove this from the standard gitignore file.

Author: pradyunsg

Python.gitignore

@@ -20,7 +20,6 @@ parts/
 sdist/
 var/
 wheels/
-pip-wheel-metadata/
 share/python-wheels/
 *.egg-info/
 .installed.cfg