lib, yang: disable libyang custom user types temporarily

lib, yang: disable libyang custom user types temporarily

libyang 1.0 introduced a few changes in the user types API, and
these changes made FRR incompatible with libyang 1.x. In order to
ease our migration from libyang 0.x to libyang 1.x, let's disable
our libyang custom user types temporarily so that FRR can work
with both libyang 0.x and libyang 1.x. This should be especially
helpful to the CI systems during the transition. Once the migration
to libyang 1.x is complete, this commit will be reverted.

Disabling our libyang custom user types should have only
minimal performance implications when processing configuration
transactions. The user types infrastructure should be more important
in the future to perform canonization of YANG data values when
necessary.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>