(bug 48480) Clean-up EmailCapture extension config

Per comment 2 in bug 48480, I'm removing EmailCapture-related
config from this file; the extension hasn't apparently been used
for a long time, and ArticleFeedback (v4) which uses this extension
is disabled globally anyway (our wikis use AFTv5).

I'll leave a potential removal of AFTv4-related code from this file
as well as from the operations/mediawiki-config repo to a new bug.

Bug: 48480
Change-Id: I3aee7b08501435f1248037eba66912436720bf4d