Remove an unused variable
Make internal logging variables static

This avoids polluting the global namespace.
Remove the unnecessary WelsSet/GetLogLevel functions

Nothing actually used the variable that these functions
handled.
Remove an unnecessary local function
Remove some commented out lines
Remove the unnecessary method WelsTraceModuleIsExist(), which always returned true
Don't include a [ENCODER]: prefix in all logging

The same trace module is used for the decoder now as well.
Merge pull request #942 from mstorsjo/log-cleanup

Clean up some of the logging/trace files