Remove redundant AssertVariableIsOfType uses in pg_upgrade

pg_upgrade code contains a number of lines like

AssertVariableIsOfType(&process_rel_infos, UpgradeTaskProcessCB);

This is presumably to ensure that the function signature is fitting for
being used with upgrade_task_add_step(). But the signature of
upgrade_task_add_step() already checks that itself, so these additional
assertions are redundant, and I find them confusing. So I propose to
remove them.

In the original thread
<https://www.postgresql.org/message-id/flat/20240516211638.GA1688936%40nathanxps13>
I found that this pattern was introduced between patch versions v9 and
v10, but there was no further explanation.