Build and test failures with Qt 5.6 and Qt 5.3

[Ivan Čukić]

Ah, for the *** sake. My fault, I expected that the 'since' tag is
missing from that one like it is for setClearColor and didn't check.

I've removed the call in question when compiling against Qt 5.3.

It will manifest as an issue for people that use dark colour scheme,
but since the activities KCM is used only by plasma which requires Qt
5.4, we should be ok.

Thanks for the reporting.

Cheerio,
Ivan

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