Don't make a11y mappings for ::checkmark

The ::checkmark pseudo-element has its contents populated via the
content property. The value of the content property can have an
additional argument provided to set the accessible name, which we
currently set to an empty string by default to hide it from the a11y
tree.

The value isn't important because I think the checked state of the
option element is already provided in a different way, and announcing a
checkmark icon is not helpful.

Based on this discussion, we should do this by default for the
::checkmark pseudo without requiring the author to also set the
accessible name to the empty string:
mdn/content#38470 (comment)

Fixed: 405946933
Change-Id: I983b1e7dff879f2c1a2184c505046d0581f93380
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6388535
Reviewed-by: Aaron Leventhal <[email protected]>
Commit-Queue: Aaron Leventhal <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1437677}

Author: josepharhar