remove INCLUDEPATH *= $$PWD from plugin .pri files

the include statements are supposed to be written in a way that makes
this unnecessary.

Change-Id: Idf0860b4c594d88477bca4528c5e9d2a172530b4
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>