make: Remove splint tests (#374)

Now that splint is actually contradicting errors that come from
the compilers I think it's time to retire it. I could cope with it
being a minor nuisance on the argument that "another check can't
hurt", but contradicting the actual compilers is too much.

The CI has Coverity installed which is much more up-to-date anyway.
Splint hasn't been updated since 2010