Remove slnx.sqlite from version control

[ErikMogensen]

Currently slnx.sqlite is part of the tracked files. It was probably added by mistake before .gitignore was added. It updates automatically by VS and thus prevent some Git commands from executing since they require all tracked files to be in the unmodified state.

It is not easy to change the Git history and there is a great answer titled Intro: You have 5 solutions available at
https://stackoverflow.com/questions/307828/how-do-you-fix-a-bad-merge-and-replay-your-good-commits-onto-a-fixed-merge.

Meanwhile the workaround is to execute: git update-index --assume-unchanged slx.sqlite before executing Git commands.

[ErikMogensen]

Fixed in #238