browsertests: disable some mail notification

Some jobs are known to be broken but still spam users.  While the root
cause is being worked on, only mail the qa-alerts mailling list.

Altered jobs:

browsertests-ContentTranslation-language-stage.wmflabs.org-linux-firefox
-sauce
browsertests-MultimediaViewer-en.wikipedia.beta.wmflabs.org-windows_7-in
ternet_explorer-8-sauce
browsertests-Translate-sandbox.translatewiki.net-linux-firefox-sauce
browsertests-UniversalLanguageSelector-language-browsertests.wmflabs.org
-linux-firefox-sauce

Bug: 70932
Change-Id: I5edd875e92da9670dc7f468af0657294b46a072e