Trim down tor-browser-build release prep issue templates
With tor-browser!559 (merged) we will have instructions for rebases and security backports in their own issue templates, so we can trim down the mega-templates in tor-browser-build.
With tor-browser!559 (merged) we will have instructions for rebases and security backports in their own issue templates, so we can trim down the mega-templates in tor-browser-build.