[test] Remove segwit argument from build_block_on_tip()

The only place that segwit=True is for a block that contains only the
coinbase transaction. Since the witness commitment is optional if none
of the transactions have a witness, we can leave it out. This doesn't
change the test coverage, which is testing p2p compact block logic.

Suggested in https://github.com/bitcoin/bitcoin/pull/20799#discussion_r867782119

test/functional/p2p_compactblocks.py

@@ -146,10 +146,8 @@ class CompactBlocksTest(BitcoinTestFramework):
         ]]
         self.utxos = []
 
-    def build_block_on_tip(self, node, segwit=False):
+    def build_block_on_tip(self, node):
         block = create_block(tmpl=node.getblocktemplate(NORMAL_GBT_REQUEST_PARAMS))
-        if segwit:
-            add_witness_commitment(block)
         block.solve()
         return block
 
@@ -381,7 +379,7 @@ class CompactBlocksTest(BitcoinTestFramework):
         # Try announcing a block with an inv or header, expect a compactblock
         # request
         for announce in ["inv", "header"]:
-            block = self.build_block_on_tip(node, segwit=True)
+            block = self.build_block_on_tip(node)
 
             if announce == "inv":
                 test_node.send_message(msg_inv([CInv(MSG_BLOCK, block.sha256)]))