Remove non-free lena-grayscale.png

[jspricke]

cf. https://lintian.debian.org/tags/license-problem-non-free-img-lenna.html

[jspricke]

as far as I've found, this image is used nowhere.

[SergioRAgostinho]

Did our CI workers got downgraded or something? it did not finish building the project under 2 hours...

[taketwo]

As far as I can see, in the last run Travis was not able to pick up pre-compiled VTK, Flann, etc. from cache and needed to recompile them.

[taketwo]

I can not find any details about when and why Travis invalidates caches, but let's assume it happened because there was not activity in the master branch for the last two months.

In the last couple of runs all the dependencies were successfully rebuilt, but since these were "pull request runs", the cache was not uploaded to the cloud. So I will merge this one to make sure that the dependencies are rebuilt and cached again.

[SergioRAgostinho]

I was gonna say to merge #1677 instead. I wanted to see if this broke any of the tests. I find it strange that we had an unused image in the repo.

[taketwo]

Too late :) But ag reveals no references to this image by name. I can't imagine that there is a test case somewhere which constructs the filename by concatenating parts.