Remove std.experimental.ndslice

[wilzbach]

I know that we scheduled ndslice removal for 2017/04, but:

1. With the ndslice docs been exposed, people aren't even aware of the deprecation
2. The deprecation messages in the build log are becoming annoying
3. 2.073 was released and contained the deprecation messages
4. The next release is scheduled to be at the end of March (?) - at least Martin hasn't even announced the end of the merge window which is supposed to be at least two weeks before a new release
5. To quote Andrei from deprecate std.experimental.ndslice #4946 "we shouldn't even need to worry with deprecations for experimental code,"

Hence I don't see any reason not to move ahead and do the final step.

[wilzbach]

(CI failures are due to DUB being down atm)

[andralex]

Yah, stuff in experimental is not expected to be stable.

[wilzbach]

Rebased this now for the third time & thus setting to "auto-merge" as it's a simple, approved package removal.