modernize: Remove custom Quassel-specific log macros

The quInfo(), quWarning(), quError() macros were introduced a long
time ago to provide custom log message handling, such as logging to
file or syslog. Such features have been directly supported for the
standard Qt log macros for a while now, using a global message handler.
The only reason for Quassel's custom macros to still exist was the
lack of an info log level prior to Qt 5.5.

As we can now rely on Qt 5.5 to be available, replace the custom
macros by Qt ones, and remove LogMessage which is no longer needed.