Skip UniqueConstraint marked by unique=True in tometadata

Skip UniqueConstraint marked by unique=True in tometadata

Fixes an issue where a Column would be copied with unique=True
and at the same time the UniqueConstraint would also be copied,
leading to duplicate UniqueConstraints in the target table,
when tometadata() is used.   Imitates the same logic used
by index=True/Index to avoid duplicates.  For some reason
a fix was implemented for Index long ago but never for
UniqueConstraint.

Change-Id: Ie622ee912a6fb8bf0ea900a8b09d78c7ebc79fc0
Fixes: #3721
(cherry picked from commit afb466fb8bd9c2f8709e79fd0fce422b83ff1d6b)