Remove unneeded translations.

Keeping these files smaller will help focus the work of translators.

We can remove most of the boilerplate translation, but still require

- including a default date format to match GOV.UK style;
- changing the units to avoid spaces between number and unit (e.g
  "121Bytes", not "121 Bytes");
- removing the Oxford comma;
- Including the faked "detailed category" name for the mainstream
  category model.
More unneeded translation keys.
Replace 'Read more' with translated text.