rts: retainer: Remove outdated invariants on traversePushStack
rts: retainer: Remove outdated invariants on traversePushStack

These invariants don't seem to make any sense in the current code.  The
text talks about c_child_r as if it were an StgClosure, for which RSET()
would make sense, but it's a retainer aka 'CostCentreStack*'.

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