Remove unused code parts

Compilation warnings:

~~~
    CC sentinel.o
sentinel.c: In function ‘sentinelRefreshInstanceInfo’:
sentinel.c:1640:9: warning: variable ‘first_runid’ set but not used [-Wunused-but-set-variable]
     int first_runid = 0;    /* true if this is the first runid we receive. */
         ^
sentinel.c:1639:9: warning: variable ‘runid_changed’ set but not used [-Wunused-but-set-variable]
     int runid_changed = 0;  /* true if runid changed. */
         ^
sentinel.c: In function ‘sentinelFailoverSwitchToPromotedSlave’:
sentinel.c:3236:9: warning: variable ‘old_master_port’ set but not used [-Wunused-but-set-variable]
     int old_master_port;
         ^
~~~