To remove just libmono-system-reactive-core2.2-cil package itself from Debian 9 (Stretch) execute on terminal:
sudo apt-get remove libmono-system-reactive-core2.2-cil
Uninstall libmono-system-reactive-core2.2-cil and it's dependent packages
To remove the libmono-system-reactive-core2.2-cil package and any other dependant package which are no longer needed from Debian Stretch.
sudo apt-get remove --auto-remove libmono-system-reactive-core2.2-cil
If you also want to delete configuration and/or data files of libmono-system-reactive-core2.2-cil from Debian Stretch then this will work:
sudo apt-get purge libmono-system-reactive-core2.2-cil
To delete configuration and/or data files of libmono-system-reactive-core2.2-cil and it's dependencies from Debian Stretch then execute:
sudo apt-get purge --auto-remove libmono-system-reactive-core2.2-cil
More information about apt-get remove
apt-get remove is identical to install except that packages are removed instead of installed. Note that removing a package leaves its configuration files on the system. If a plus sign is appended to the package name (with no intervening space), the identified package will be installed instead of removed.