CiviMailUtils::clearMessages() should clear all messages by default

Currently, CiviMailUtils::clearMessages() clears one message from the message log per default.

The function docs say "Remove any sent messages from the log.", but the default $limit value is 1.

• This means that the function name and docs are not consistent with the behaviour.
• This also leads to developers putting arbitrary-but-large numbers in tests ($mut->clearMessages(999)), which introduces more WTF ("What is the 999 here?") and is not guaranteed to clear all messages.

I propose that we set the default $limit to 0 instead, meaning that the behaviour is more consistent with the function naming and docs, and that code which actually requires a single message to be removed just use $mut->clearMessages(1).

Whether this is an external interface might affect whether this is a behaviour change in CiviCRM?