[Doc] Remove/Improve build-breaking "Dummy" method docs

[nephros]

Turns out these methods are not so dummy after all. (See #452)

One is a Signal, which shouldn't have an implementation at all.

The other is indeed dead code, but lack of usage o its parameter broke the
build. Instad of wrapping it in Q_UNUSED, lets just not have an
implementation, just the qdoc comment.

This now does silence qdoc, and does not break the build.

[Olf0]

Will look at this, try to comprehend the storyline which developed over years and approve, soon™.

[nephros]

Hey @Olf0 if you have the time it would be nice if you could approve and merge this.

The issue currently prevents building master on OBS.

[Olf0]

IMO this confirms to stay away from any "easter shit" in the first place: It is additional, superfluous code, which causes a maintenance burden. 😛
Well, never mind my ranting.

Seriously:
I do not fully comprehend why you have taken out the textual pointers to the function declarations in the corresponding header files and to the locations these functions are used, which were intended not to loose track of these places which will have to be tackled when fully eliminating this unnecessary code. Knowing GitHub-search better now, I know they are easy to find again, but still someone will have to perform this (re)search again, then.

Still I approve this PR as I do not have an overly strong opinion about this, but I would be glad if you reconsider keeping the detailed comments in or explicitly name a reason why you believe they must go.