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"