on_cancel pre-blocked cancellation with its own "Purchase Invoice is
already submitted" guard, duplicating the check Frappe already runs for any
submitted linked document. Drop the guard and the unused check_next_docstatus()
method it mirrored so the receipt defers to the framework: the Cancel All
Documents flow cancels the invoice first and then the receipt, and a direct
cancel is still rejected by Frappe's linked-document check.
Add a regression test that a direct cancel of a receipt with a submitted
invoice is rejected and rolls back, leaving no stray stock or GL entries.
cfe18e8427