The code maintains the invariant that raw and opaque PSKs are never
configured simultaneously, so strictly speaking `ssl_conf_remove_psk()`
need not consider clearing the raw PSK if it has already cleared an
opaque one - and previously, it didn't. However, it doesn't come at
any cost to keep this check as a safe-guard to future unforeseen
situations where opaque and raw PSKs _are_ both present.
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