Revert "adding entry for ignoring <solutionname>.sln.ide folder which co...

...ntains visual studio 2015 solution specific cache data"

This reverts commit b07a0904e3c0714353f997b1dc6122b367dab648

There's an entry for `*.ide/` and I cannot see what the point of `*.sln.ide/` is.

https://github.com/github/gitignore/blob/master/VisualStudio.gitignore#L26.