Remove hardwired inclusion of OSG

The remainder of .pro and .pri files have conditional inclusion of
OpenSceneGraph bases on availability. Removed this line which always
includes it and overrides the conditional include which is already in
other parts of the make files.

qgroundcontrol.pro

@@ -875,8 +875,6 @@ win32-msvc2008|win32-msvc2010|win32-msvc2012 {
     DEFINES += MOUSE_ENABLED_WIN
 }
 
-unix:!macx:!symbian: LIBS += -losg
-
 OTHER_FILES += \
     dongfang_notes.txt \
     src/ui/dongfang-scrapyard.txt \