[WCJS] Remove `:unresolved` pseudo selector support

[TimvdLippe]

This feature did not make the v1 specification and therefore we should remove the support from the polyfill. It appears that :defined is the successor, but that is exactly the opposite of :unresolved.

Removal of this feature solves CSP compatibility and would thus resolve #188 and #969.

[dfreedm]

This is the opposite solution of the one proposed in #51.

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This issue has been automatically closed after being marked stale. If you're still facing this problem with the above solution, please comment and we'll reopen!