Fix crash when multiple MySQL monitors monitor same servers

The monitors always freed and reallocated the memory for the slaves. It
was always of the same size so a static array of that size should also
work.

server/include/monitor.h

@@ -129,7 +129,6 @@ typedef enum
 
 #define MONITOR_INTERVAL 10000 // in milliseconds
 #define MONITOR_DEFAULT_ID 1UL // unsigned long value
-#define MONITOR_MAX_NUM_SLAVES 20 //number of MySQL slave servers associated to a MySQL master server
 
 /*
  * Create declarations of the enum for monitor events and also the array of