Remove support for Qt 4 style plugins

Remove support for Qt 4 style plugins

The new plugin format allows us to avoid loading the plugins in
all cases. Remove the old format, as we could get bad behavior
with the old format if Qt would try to dlopen a Qt 4.x plugin.

Change-Id: I2193e6874d6cca3c0b12298c2b9beb4105a42fd5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>