Remove old, unused LB instrumentation data structures

[lifflander]

Remove all the old load balancing infrastructure that is not used. When I started implementing LB, I sketched out some implementation in vt/src/vt/lb/*. I believe none of this code is active or used. Try to remove it and ensure none of it is used anywhere.

[JacobDomagala]

It seems that only vt/src/vt/lb/migration/lb_migratable.h is used (HasMigrate struct is base struct for UntypedCollection) I suggest we either move the file or the HasMigrate struct declaration to vt/src/vt/vrt/collection* directory.

NOTE: There's mention of this interface in vt/src/vt/vrt/collection/types/migratable.h:91 together with explanation how it's supposed to be used. Not sure if we want to touch it, just wanted to point that out since there's @todo added to it.

What's your opinion @lifflander @PhilMiller