Remove useless tests for TRUNCATE on foreign tables

Remove useless tests for TRUNCATE on foreign tables

foreign_data has kept around a set of tests for TRUNCATE to look after
the case of foreign tables, with[out] inheritance and with[out]
partitions, assuming that the command is not supported for this relkind.
However, TRUNCATE is supported on foreign tables if the FDW involved is
able to handle the command, like postgres_fdw.

Note that postgres_fdw includes tests to cover all the cases removed by
this commit (which had misleading comments), so these did not provide
any additional coverage anyway.

Author: Yugo Nagata
Discussion: https://postgr.es/m/20220527172543.0a2fdb469cf048b81c0967d3@sraoss.co.jp