Forminator is an excellent WordPress forms plugin. Follow these steps to remove the default shadow hover effect on a forms submit button.

1. Login to your WordPress site admin.
2. Go to Forminator Pro > Forms
3. Click to edit a form.
4. Click the Appearance link (top-left).
5. Scroll down and click the Enable Custom CSS toggle button.
6. Paste the CSS code (see below) into the custom CSS field.
7. Click the Update button (top-right) to save your edits.
8. Click the Preview button (top-right) to preview the form.

The CSS Code:
button.forminator-button.forminator-button-submit {
-webkit-box-shadow: none !important;
-moz-box-shadow: none !important;
box-shadow: none !important;
}

Enjoy!