SMWHooks::addPoweredBySMW clean-up

+ Remove addPoweredBySMW hook because it is not being used, the icon
is set by $wgFooterIcons [1]
+ Use https instead of http url

[1] http://www.mediawiki.org/wiki/Manual:$wgFooterIcons

Change-Id: Ib7cdbac0ec940013c298a77f3759d8c83080125e