In addition to Leon's advice (first option - this suggestion is second), try going directly to
yoursite/admin/plugins.php
via browser. If you are not logged on it will prompt to login and then redirect to the listing of plugins where one can find 'uninstall' links.
In addition, the plugin you mentioned is part of a set and on it's info page:
https://moodle.org/plugins/plagiarism_turnitin
says
"This plugin shares code with the new mod_turnitintool2 plugin which must be installed at the same time."
Your way out might be to manually install the 'mod_turnitintool2 plugin' which can be found:
https://moodle.org/plugins/browse.php?list=set&id=52
2nd plugin listed at url above.
'SoS', Ken