PEP 554: Remove orphan main-thread citation

Author: hugovk

pep-0554.rst

@@ -1678,10 +1678,6 @@ References
 .. [benefits]
    https://mail.python.org/pipermail/python-ideas/2017-September/047122.html
 
-.. [main-thread]
-   https://mail.python.org/pipermail/python-ideas/2017-September/047144.html
-   https://mail.python.org/pipermail/python-dev/2017-September/149566.html
-
 .. [reset_globals]
    https://mail.python.org/pipermail/python-dev/2017-September/149545.html