[BUGS-6657] Handle explicit WP_ALLOW_MULTISITE false correctly

[pwtyler]

Sites with define('WP_ALLOW_MULTISITE', false); are erroneously getting prompted to fix their upstream, as we we only check WP_ALLOW_MULTISITE was set, not its value.

Further consolidated the logic given we were checking both WP_ALLOW_MULTISITE and MULTISITE back-to-back

[pwtyler]

Pushed patch to a test site with define('WP_ALLOW_MULTISITE', false); and asserted the banner disappears.

Before

After

[jazzsequence]

This looks correct to me. My only question (and the answer is probably "no") is whether the splitting of the two things was intentional and whether there's ever a case where WP_ALLOW_MULTISITE is defined but not true and MULTISITE is not defined or is empty and we'd want to load both files.

I think this is a wild enough idea that it's unlikely it was ever the intended purpose so, without looking into each of those files, I'm gonna go ahead and just say yeah, do it.