Supress native "invalid email" warning on Special:ChangeEmail

To avoid a double "invalid email" warning (one from the browser,
one from updateMailValidityLabel), prevent the browser's  "invalid
email" notice from appearing and just run updateMailValidityLabel,
which generates its own notice.

Bug: 40909
Change-Id: Id657c6ce2e72d877510fcf690aef2c548d8cdf1d
Merge "Supress native "invalid email" warning on Special:ChangeEmail"