Remove unused media_url_prefix from project_downloads template context
Remove unused media_url_prefix from project_downloads template context

Fixes #1605.

sunnyzwh/readthedocs.org,attakei/readthedocs-oauth,stevepiercy/readthedocs.org,safwanrahman/readthedocs.org,laplaceliu/readthedocs.org,tddv/readthedocs.org,wijerasa/readthedocs.org,davidfischer/readthedocs.org,pombredanne/readthedocs.org,wijerasa/readthedocs.org,techtonik/readthedocs.org,rtfd/readthedocs.org,safwanrahman/readthedocs.org,sunnyzwh/readthedocs.org,espdev/readthedocs.org,SteveViss/readthedocs.org,laplaceliu/readthedocs.org,stevepiercy/readthedocs.org,davidfischer/readthedocs.org,rtfd/readthedocs.org,stevepiercy/readthedocs.org,tddv/readthedocs.org,espdev/readthedocs.org,gjtorikian/readthedocs.org,gjtorikian/readthedocs.org,espdev/readthedocs.org,istresearch/readthedocs.org,istresearch/readthedocs.org,rtfd/readthedocs.org,sunnyzwh/readthedocs.org,emawind84/readthedocs.org,istresearch/readthedocs.org,techtonik/readthedocs.org,laplaceliu/readthedocs.org,gjtorikian/readthedocs.org,emawind84/readthedocs.org,gjtorikian/readthedocs.org,clarkperkins/readthedocs.org,clarkperkins/readthedocs.org,sunnyzwh/readthedocs.org,wijerasa/readthedocs.org,emawind84/readthedocs.org,safwanrahman/readthedocs.org,safwanrahman/readthedocs.org,emawind84/readthedocs.org,attakei/readthedocs-oauth,davidfischer/readthedocs.org,rtfd/readthedocs.org,attakei/readthedocs-oauth,techtonik/readthedocs.org,istresearch/readthedocs.org,techtonik/readthedocs.org,davidfischer/readthedocs.org,espdev/readthedocs.org,SteveViss/readthedocs.org,pombredanne/readthedocs.org,pombredanne/readthedocs.org,clarkperkins/readthedocs.org,laplaceliu/readthedocs.org,clarkperkins/readthedocs.org,attakei/readthedocs-oauth,tddv/readthedocs.org,wijerasa/readthedocs.org,SteveViss/readthedocs.org,espdev/readthedocs.org,SteveViss/readthedocs.org,stevepiercy/readthedocs.org