htmlform: Fix OOUI checkmatrix header cell padding

The padding was not applied to header cells, causing their text
to clash with each other. While I'm here, adjust other spacing
minutely to match Codex table design and remove unused CSS.

This component is used in the Echo extension's preferences,
and the English-language labels are short enough that the
problem wasn't apparent. Here's the same component in Polish:
Before: https://phabricator.wikimedia.org/F57276954
After: https://phabricator.wikimedia.org/F57276955

Change-Id: I5051e93a1f4e59d5aabeaf04f09ce92520735f3d