Remove hx_Subhead--responsive

[simurai]

This is part of https://github.com/github/primer/issues/322 and removes the hx_Subhead--responsive class.

Subhead is now responsive by default and this class is not needed anymore.

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[manuelpuyol]

@jonrohan do we have a plan to update to PrimerCSS 18? Some color variables changed and will break our utilities

[jonrohan]

@jonrohan do we have a plan to update to PrimerCSS 18? Some color variables changed and will break our utilities

I was hoping we could get the release out with 17.11.1 first so we have time to migrate. It was merged in #858

[simurai]

@manuelpuyol do we have a plan to update to PrimerCSS 18? Some color variables changed and will break our utilities

@jonrohan I was hoping we could get the release out with 17.11.1 first so we have time to migrate. It was merged in #858

Actually, I think we don't have to wait for PrimerCSS 18. Because the hx_Subhead--responsive styles got removed in https://github.com/github/github/pull/198706 on dotcom and they were never part of PrimerCSS. So I guess this PR could already be merged since having hx_Subhead--responsive doesn't really do anything anymore.