Fix qmake warning about "Redundant entries in QTPLUGIN"

Fix qmake warning

Redundant entries in QTPLUGIN: qico qtaccessiblewidgets

In qtbase commit 4255ba40ab073af (Qt 5.3.0 and newer)
the plugins are added automatically, based on the Qt module
one links against.

Change-Id: I45200db2f3bf9b6194f92bf864ab58ceb5c71f16
Reviewed-by: Niels Weber <niels.weber@digia.com>