minor #13940 Remove links to not existing channel pages (ThomasTr)

This PR was submitted for the master branch but it was merged into the 5.0 branch instead.

Discussion
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Remove links to not existing channel pages

Remove dead links to not existing (sms|chat|email)-channel pages

Commits
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02ee00b Remove links to not existing channel pages

Author: javiereguiluz

notifier.rst

@@ -31,10 +31,10 @@ by using transports.
 
 The notifier component supports the following channels:
 
-* `SMS <SMS Channel>`_ sends notifications to phones via SMS messages
-* `Chat <Chat Channel>`_ sends notifications to chat services like Slack
+* SMS sends notifications to phones via SMS messages
+* Chat sends notifications to chat services like Slack
   and Telegram;
-* `Email <Email Channel>`_ integrates the :doc:`Symfony Mailer </mailer>`;
+* Email integrates the :doc:`Symfony Mailer </mailer>`;
 * Browser uses :ref:`flash messages <flash-messages>`.
 
 .. tip::