Update _button_mixins.scss to remove underline for users not includin…

…g reset.css (#3390)

**What does this PR do?** A clear and concise description with your
runway ticket url.

- Update _button_mixins.scss to remove underline for users not including
reset.css

Co-authored-by: Mark Rosenberg <38965626+markdoeswork@users.noreply.github.com>

playbook/app/pb_kits/playbook/pb_button/_button_mixins.scss

@@ -28,6 +28,7 @@ $pb_button_border_width: 0px;
   font-size: $font_small;
   font-weight: $bold;
   text-align: center;
+  text-decoration: none;
   vertical-align: middle;
   text-transform: none;
   border-width: $pb_button_border_width;