[bot] Remove obsolete l10n strings and ftl files

### Remove obsolete strings
Search the code base for [strings marked `obsolete`](https://github.com/search?q=repo%3Amozilla%2Fbedrock+obsolete+language%3AFluent&type=code&l=Fluent) 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 and their fallback usage
### 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

Start by searching the bedrock code base for Fluent strings marked obsolete and compare /l10n/en-US/ with www-l10n/en. Use git history to date undated strings, and run the provided diff command if both repositories are available. Done means old strings and fallback usage are removed, obsolete files are removed from www-l10n, and their references are removed from www-l10n/configs/pontoon.toml.

Written by the indexing model from the issue text.