Remove redundant AssertVariableIsOfType uses

Remove redundant AssertVariableIsOfType uses

The uses of AssertVariableIsOfType in pg_upgrade are unnecessary
because the calls to upgrade_task_add_step() already check the
compatibility of the callback functions.

These were apparently copied from a previous coding style, but similar
removals were already done in commit 30b789eafe.

Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/3d289481-7f76-409f-81c7-81824219cc75%40eisentraut.org