As the truncate only option has been removed by Microsoft to avoid misuse of the command, there still is a workaround to shrink the transcation log without having to actually back it up.
The trick is to backup the log to a nul disk as shown below
BACKUP LOG MicrosoftDynamicsAX_model TO DISK='NUL:'
dbcc shrinkfile( MicrosoftDynamicsAX_model_log, 0 )
BACKUP LOG MicrosoftDynamicsAX TO DISK='NUL:'
dbcc shrinkfile( MicrosoftDynamicsAX_log, 0 )