fbt: Remove some handling for multiple CTF containers

This was ported from illumos but not completely done. Currently we do
not perform type deduplication between KLDs and the kernel, i.e., kernel
modules have a complete type graph. So, remove it for now since it's
not functional and complicates the task of modifying various CTF type
definitions, and we are hitting some limits in the current format which
necessitate an update.

No functional change intended.

MFC after: 2 weeks