Remove nifti header builder

[Enet4]

This removes the NiftiHeaderBuilder and the dependency derive_builder. Reasons:

• It was not very useful, considering that we can build a default instance and work from there.
• Error handling isn't ideal, the error type is String which does not match how errors emerge from the rest of the crate, and I'm not sure if this would ever return an error, considering that all fields have a default value.
• The builder could leave the false impression that fields are validated when using it, which was never really the case.
• I could not just deprecate it because the implementation is behind a derive macro.

Hopefully there's no one relying heavily on this builder.

[nilgoyette]

I don't know about other programmers, but I confirm that we never used it. LGTM.