Remove unused variable from conf.py

Author: tony

doc/conf.py

@@ -36,7 +36,6 @@
 releases_document_name = ["history"]
 releases_issue_uri = "https://github.com/tmux-python/libtmux/issues/%s"
 releases_release_uri = "https://github.com/tmux-python/libtmux/tree/v%s"
-releases_future_release_uri = "https://github.com/tmux-python/libtmux/tree/%s"
 
 issues_github_path = about['__github__'].replace('https://github.com/', '')