Pip install fails because of --use-mirrors parameter

Having --use-mirrors parameter in pip commands causes pip to hang on
some distros.

Pypi uses CDN for long time already, so there's no point to keep this
parameter no more.

Wipe PIP_USE_MIRRORS out of the "function" file.

Change-Id: I70adaf6591834af2482e09eb7f8f9f60df8e7692
Closes-Bug: #1069309
Merge "Pip install fails because of --use-mirrors parameter"