noui: Remove always empty caption while formatting

The only behavior change is in noui_ThreadSafeQuestion, which can not
detect a style and will log a strCaption=": ".

Fix this by removing it.

test/functional/rpc_blockchain.py

@@ -124,7 +124,7 @@ class BlockchainTest(BitcoinTestFramework):
         self.log.info("A block tip of more than MAX_FUTURE_BLOCK_TIME in the future raises an error")
         self.nodes[0].assert_start_raises_init_error(
             extra_args=[f"-mocktime={TIME_RANGE_TIP - MAX_FUTURE_BLOCK_TIME - 1}"],
-            expected_msg=": The block database contains a block which appears to be from the future."
+            expected_msg="The block database contains a block which appears to be from the future."
             " This may be due to your computer's date and time being set incorrectly."
             f" Only rebuild the block database if you are sure that your computer's date and time are correct.{os.linesep}"
             "Please restart with -reindex or -reindex-chainstate to recover.",