Remove obsolete l10n strings and ftl files

[github-actions]

Remove obsolete strings

Search the code base for strings marked obsolete that are more than 2 months old.
If you find strings that have no date, either find the date by looking at git history
or add an expiry date and leave it to be removed in 2 months.

• Remove old obsolete strings

Remove obsolete ftl files

Compare the files in /l10n/en-US/ to the /en/ folder in the www-l10n repo and remove
files from www-l10n that no longer exist in bedrock.

If you have both repos installed and up to date you can run this command:
diff -r www-l10n/en bedrock/l10n/en | grep ".ftl"

• Remove files
• Remove references in www-l10n/configs/pontoon.toml