Remove using namespace directive in a header

[nedseb]

A using namespace inside a header makes useless namespace so it should be removed.

[finneyj]

Thanks @nedseb.

I agree that this using directive shouldn't be here. I seem to recall that it was a "quick fix" necessary to support transparency for microbit-v2 (out biggest user). I think there are also other directives like this in other .h files that we should remove at the same time. Doing so will require us to ensure that we don't introduce incompatibilities though...

[nedseb]

Yes, I understand....
In the beginning, I made this PR because I want to see if this change will break the CI to know if this line was here for a good reason.