Move Mapgen V7 river generation into the main generation loop (#10639)

Move Mapgen V7 river generation into the main generation loop (#10639)

All terrain generation now occurs in one loop, instead of rivers being
carved afterwards in a separate loop.
Fixes the removal of nodes added by mods in 'register on generated'.
Avoids other problems and reduces the chance of future bugs.
Mapchunk generation time is reduced.

Also fixes a code mistake which resulted in river channel generation
being disabled at floatland altitudes even when floatlands were disabled.