Just deactivate the plugin and your site will work the same as before installation.
The WP Google Search plugin uses the “?q=” parameter, not “?s=”.
The “?s=” parameter is used by WordPress’s built-in search engine (not Google search engine), so it worked before the WP Google Search plugin was installed and will remain alive after you deactivate it.
“?s=” never redirect you to the gcse_results page (I tried this on our own test sites).
I don’t know the “?text=whatever” parameter.
If you need more information, please send us the link of your site where the issue can be seen.