histedit: removing the experimental config 'histeditng' This config has been around for about 2 years now. So, it can be assumed to be stable. Therefore, I am removing the config. This also makes the test 'test-histedit-base.t' compatible with chg. Test Plan: - Ran the test 'test-histedit-base.t' with and without '--chg' option. - Ran all the other tests without '--chg' option. - Checked the output of 'hg help histedit'. Differential Revision: https://phab.mercurial-scm.org/D942