Remove dropped v3 bootstrap-theme

[ci skip]

Author: vsn4ik

.gitattributes

@@ -12,6 +12,5 @@
 # Don't diff or textually merge source maps
 *.map   binary
 
-bootstrap-theme.css linguist-vendored=false
 bootstrap.css linguist-vendored=false
 bootstrap.js linguist-vendored=false

CONTRIBUTING.md

@@ -135,7 +135,7 @@ Please adhere to the [coding guidelines](#code-guidelines) used throughout the
 project (indentation, accurate comments, etc.) and any other requirements
 (such as test coverage).
 
-**Do not edit `bootstrap.css`, `bootstrap-theme.css`, or `bootstrap.js`
+**Do not edit `bootstrap.css`, or `bootstrap.js`
 directly!** Those files are automatically generated. You should edit the
 source files in [`/bootstrap/scss/`](https://github.com/twbs/bootstrap/tree/master/scss)
 and/or [`/bootstrap/js/`](https://github.com/twbs/bootstrap/tree/master/js) instead.