Doc: removed unnecessary page about plugins.

The "All QML APIs by Module" replaced this page.
The group still exists, but the members don't link to it.

Task-number: QTBUG-32580
Change-Id: Iabab2b1330f3b20e142009e42a2a4aad63af2c72
Reviewed-by: Martin Smith <martin.smith@digia.com>
Doc: Removed unnecessary files

-nothing in qtdoc repository uses them
-images, CSS, .qdocconf, and other files

Change-Id: I23a947a3731937ff604debded29535472881e14b
Reviewed-by: Martin Smith <martin.smith@digia.com>