Formally deprecate "AbstractGraphicsContext.get_full_text_extent"

[rahulporuri]

https://github.com/enthought/enable/blob/3d138a235547240c98ed8aba1046a3f1c016d42b/kiva/abstract_graphics_context.py#L515-L523

AbstractGraphicsContext.get_full_text_extent has been deprecated in favor of AbstractGraphicsContext.get_text_extent but the deprecation is just on paper because we don't actually raise any warnings. We also haven't removed any uses of the deprecated get_full_text_extent method.

Note that the method was created and documented as deprecated in the PR https://github.com/enthought/enable/pull/139 - but its not clear why.