Remove set_latency_target(), add lock_memory_to_ram()

set_latency_target() attempts to set keep "low latency" by
- writing a specific latency to /dev/cpu_dma_latency
- setting manually the minimum CPU processor frequency to be the maximum

There is no functionality to undo this after stopping the softmodem(s),
and most users are probably not even aware that OAI does this. It is
generally preferable to set this beforehand using a governor or by
disabling sleep states (as outlined in the tutorial, and in a follow-up
commit in the performance tuning docs).

The previous mlockall() call, to lock memory to RAM, is retained in a
new function. There were additional mlockall() calls, which have been
replaced with lock_memory_to_ram(), where necessary.

common/utils/system.h

@@ -42,7 +42,7 @@ int background_system(char *command);
 
 void start_background_system(void);
 
-void set_latency_target(void);
+void lock_memory_to_ram(void);
 
 void threadCreate(pthread_t *t, void *(*func)(void *), void *param, char *name, int affinity, int priority);