sql: Fix prefix stats for large IPv6 prefixes
sql: Fix prefix stats for large IPv6 prefixes

Large IPv6 prefixes results in very large numbers of used or free
addresses and since the num_used variable used was a plain integer it
couldn't handle such large numbers. Now changed to numeric!

Also remove an unused variable.

garberg/NIPAP,fredsod/NIPAP,garberg/NIPAP,fredsod/NIPAP,ettrig/NIPAP,ettrig/NIPAP,bbaja42/NIPAP,SoundGoof/NIPAP,SpriteLink/NIPAP,SoundGoof/NIPAP,bbaja42/NIPAP,plajjan/NIPAP,SpriteLink/NIPAP,plajjan/NIPAP,SpriteLink/NIPAP,garberg/NIPAP,fredsod/NIPAP,plajjan/NIPAP,garberg/NIPAP,SoundGoof/NIPAP,SpriteLink/NIPAP,bbaja42/NIPAP,plajjan/NIPAP,ettrig/NIPAP,fredsod/NIPAP,fredsod/NIPAP,SoundGoof/NIPAP,SpriteLink/NIPAP,fredsod/NIPAP,ettrig/NIPAP,SoundGoof/NIPAP,plajjan/NIPAP,plajjan/NIPAP,ettrig/NIPAP,SoundGoof/NIPAP,bbaja42/NIPAP,bbaja42/NIPAP,bbaja42/NIPAP,garberg/NIPAP,SpriteLink/NIPAP,ettrig/NIPAP,garberg/NIPAP