Remove obsolete "Firefox account" string

Author: Vinnl

locales/en/app.ftl

@@ -11,7 +11,6 @@
 -product-short-name = Monitor
 -brand-name = Firefox
 -brand-HIBP = Have I Been Pwned
--brand-fxa = Firefox Account
 -brand-lockwise = Firefox Lockwise
 -brand-firefox = Firefox
 -brand-monitor = Monitor
@@ -53,9 +52,6 @@ show-all = Show All
 
 sign-out = Sign Out
 
-# Manage Firefox Account, link to page where account holders can change their account settings.
-manage-fxa = Manage { -brand-fxa }
-
 # Link title
 preferences = Preferences