Assert that partition inherits from only one parent in MergeAttributes()
A partition inherits only from one partitioned table and thus inherits
a column definition only once. Assert the same in MergeAttributes()
and simplify a condition accordingly.
Similar definition exists about line 3068 in the same function.
Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAExHW5uOykuTC+C6R1yDSp=o8Q83jr8xJdZxgPkxfZ1Ue5RRGg@mail.gmail.com