efl_mono: remove class ExtensionTag

Summary:
Since class BindablePart and class BindableProperty use generic type
constraint as "where T : <class name>", class ExtensionTag is no longer
needed.

Reviewers: SanghyeonLee, felipealmeida, YOhoho

Reviewed By: YOhoho

Subscribers: cedric, reviewers, committers

Tags: efl

Differential Revision: https://phab.enlightenment.org/D11101