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/feature_presegwit_node_upgrade.py

@@ -38,7 +38,7 @@ class SegwitUpgradeTest(BitcoinTestFramework):
         # because the blockchain consists of 3 insufficiently validated blocks per segwit consensus rules.
         node.assert_start_raises_init_error(
             extra_args=["-testactivationheight=segwit@5"],
-            expected_msg=": Witness data for blocks after height 5 requires "
+            expected_msg="Witness data for blocks after height 5 requires "
             f"validation. Please restart with -reindex..{os.linesep}"
             "Please restart with -reindex or -reindex-chainstate to recover.",
         )

test/functional/feature_reindex_init.py

@@ -21,7 +21,7 @@ class ReindexInitTest(BitcoinTestFramework):
         self.log.info("Removing the block index leads to init error")
         shutil.rmtree(node.blocks_path / "index")
         node.assert_start_raises_init_error(
-            expected_msg=f": Error initializing block database.{os.linesep}"
+            expected_msg=f"Error initializing block database.{os.linesep}"
             "Please restart with -reindex or -reindex-chainstate to recover.",
         )
 

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.",