removal proposal: mongodb3

[Greg Troxel <gdt%lexort.com@localhost>]

The last upstream release of mongodb3 was over 6 years ago.  It exists
in pkgsrc because it's the last Free Software version.  unifi used to
use it.

It's the kind of thing that people feel the need to fix for newer
compilers, from bulk build breakage.  Looking at recent changes to
patches, they seem to be in the fix-with-newer-compiler category.

I suggest that there are no users, and that it's time to remove it (and
drop from meta and bulk-test)

Unless there are objections, I'll remove it no earlier than 17 July,