Bindings: Remove INSTALL_DIR_SUFFIX from ecm_generate_python_binding

Summary:
The option is no longer used from any framework, and was always defaulting
to site-packages previously, as INSTALL_DIR_SUFFIX was empty.

Also see D15558, D15845.

Test Plan: build e.g kcoreaddons

Reviewers: Frameworks, apol, lbeltrame

Reviewed By: lbeltrame

Subscribers: lbeltrame, kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D15867