relax child-with-unconfirmed-parents rule

This rule was originally introduced along with a very early proposal for
package relay as a way to verify that the "correct"
child-with-unconfirmed-parents package was provided for a transaction,
where correctness was defined as all of the transactions unconfirmed
parents. However, we are not planning to introduce a protocol where
peers would be asked to send these packages.

This rule has downsides: if a transaction has multiple parents but only
1 that requires package CPFP to be accepted, the current rule prevents
us from accepting that package. Even if the other parents are already in
mempool, the p2p logic will only submit the 1p1c package, which fails
this check. See the test in p2p_1p1c_network.py

test/functional/p2p_1p1c_network.py

@@ -101,7 +101,7 @@ class PackageRelayTest(BitcoinTestFramework):
         package_hex_2, parent_2, child_2 = self.create_basic_1p1c(self.wallet_nonsegwit)
 
         # 3: 2-parent-1-child package. Both parents are above mempool min feerate. No package submission happens.
-        # We require packages to be child-with-unconfirmed-parents and only allow 1-parent-1-child packages.
+        # We require packages to be child-with-parents and only allow 1-parent-1-child packages.
         package_hex_3, parent_31, _parent_32, child_3 = self.create_package_2p1c(self.wallet)
 
         # 4: parent + child package where the child spends 2 different outputs from the parent.