refactor: Remove unused constants

[ftnext]

Thank you for this awesome library.
I referred to the implementation of the octicon role in icons.py (a3bdfa3).

In reading the implementation, I noticed some unused code, so I propose to remove it.

$ git grep OCTICON_CSS
sphinx_design/icons.py:OCTICON_CSS = """\
$ git grep OCTICON_VERSION
sphinx_design/icons.py:OCTICON_VERSION = "v19.8.0"

[chrisjsewell]

Thanks @ftnext for the tidy-up, and apologies it sat for so long! Your change has landed on main via #266 (81dbab8) with your commit authorship preserved — the original branch here couldn't re-trigger the status checks that are now required (the 2024 CI runs were too old to re-run), so it was re-created rather than merged directly. Closing this one. 🙏

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