[transfer-engine] Remove additional doc install rule from doc.pri.

qdoc seems to handle documentation installation itself. This install rule
installed a second copy of documentation to a different path.

How this worked previously, I am not entirely sure...

doc/doc.pri

@@ -13,7 +13,3 @@ equals(QT_MAJOR_VERSION, 5): docs.commands = (cd doc/config; $$QDOC $$QDOCCONF;
 
 QMAKE_EXTRA_TARGETS += docs
 
-doc.files = doc/html/*
-doc.path = /usr/share/doc/nemo-transfer-engine
-INSTALLS += doc
-