Remove S60 plugins from code and installation packages

3.1, 3.2 and 5.0 support has been made with creating plugin variants for each platform version.
From 4.7.3 onwards 3.x isn't supported anymore and thus we can remove the additional DLL files from sis package and also reduce the complexity of code in QtCore and plugins. Location is \src\plugins\s60 .

Note this applies to QtMobility as well, distributed sis file sizes can be greatly reduced with removing the extra DLLs. Maybe a separate task for QtMobility is in place?