docs: remove credential helper links for emails from gitcredentials

docs: remove credential helper links for emails from gitcredentials

In a recent attempt to add links of email helpers to git-scm.com [1], I
came to a conclusion that the links in the gitcredentials page are meant
for people needing credential helpers for cloning, fetching and pushing
repositories to remote hosts, and not sending emails. gitcredentials
docs don't even talk about send emails, thus confirming this view.

So, lets remove these links from the gitcredentials page. The links are
still available in the git-send-email documentation, which is the right
place for them.

[1]: https://github.com/git/git-scm.com/pull/2005

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>