Bug 1551653 - Remove prefs_content_discovery_description string from locale file r=fluent-reviewers,pdahiya,flod

Cleaning up prefs_content_discovery_description that we migrated over to fluent just in case we still wanted it.

Differential Revision: https://phabricator.services.mozilla.com/D37948

--HG--
extra : moz-landing-system : lando

browser/locales/en-US/browser/preferences/preferences.ftl

@@ -502,7 +502,6 @@ choose-bookmark =
 
 home-prefs-content-header = Firefox Home Content
 home-prefs-content-description = Choose what content you want on your Firefox Home screen.
-home-prefs-content-discovery-description = Content Discovery in Firefox Home allows you to discover high-quality, relevant articles from across the web.
 
 home-prefs-search-header =
     .label = Web Search