Remove use_ozone=false from flags.gn

There's no reason for it to be explicitly disabled. Also, Ozone could be
beneficial in the future when it becomes the default on desktop Linux
for Wayland users.

See https://github.com/Eloston/ungoogled-chromium/issues/724#issuecomment-510961997

flags.gn

@@ -29,5 +29,4 @@ safe_browsing_mode=0
 symbol_level=0
 treat_warnings_as_errors=false
 use_official_google_api_keys=false
-use_ozone=false
 use_unofficial_version_number=false