Revert "[core] Add mysqlclient 2.1.1 for Python 3 (#3339)"

This reverts commit 0a5f7aeeab7c88022ccd0a5dcaab89af9abcb093.

desktop/core/base_requirements.txt

@@ -38,7 +38,6 @@ kerberos==1.3.0
 lockfile==0.12.2
 Mako==1.2.3
 Markdown==3.1
-mysqlclient==2.1.1 # don't move ext-py3
 nose==1.3.7
 openpyxl==3.0.9
 phoenixdb==1.2.1