Clean up opentracing configuration options (#5712)

Clean up config settings and dead code.

This is mostly about cleaning up the config format, to bring it into line with our conventions. In particular:
 * There should be a blank line after `## Section ##' headings
 * There should be a blank line between each config setting
 * There should be a `#`-only line between a comment and the setting it describes
 * We don't really do the `#  #` style commenting-out of whole sections if we can help it
 * rename `tracer_enabled` to `enabled`

While we're here, do more config parsing upfront, which makes it easier to use
later on.

Also removes redundant code from LogContextScopeManager.

Also changes the changelog fragment to a `feature` - it's exciting!

synapse/logging/scopecontextmanager.py

@@ -34,9 +34,7 @@ class LogContextScopeManager(ScopeManager):
     """
 
     def __init__(self, config):
-        # Set the whitelists
-        logger.info(config.tracer_config)
-        self._homeserver_whitelist = config.tracer_config["homeserver_whitelist"]
+        pass
 
     @property
     def active(self):