chore: simplify deriveds a tiny bit

[Rich-Harris]

While wrapping my head round deriveds for the sake of #12047 I started to wonder if there were some other opportunities for simplification. Baby steps so far:

• the return !is_equal line can just be return true, because is_equal is guaranteed to be false in the case that the dependency version is higher than the current reaction version
• by extension, we don't need to return a boolean from update_derived, and can simplify that function slightly
• (perhaps because of fix: increment derived versions when updating #12047) the second half of if (is_dirty && !is_unowned) appears to be unnecessary

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[dummdidumm]

Makes sense - and yes you're right, the unowned check can be removed because no longer synchronize the versions, instead we just increment a global version upon updating deriveds now which handles this.