Remove cpu_iowait

The third value from load_avg (idle) gives us almost the same
information as cpu_iowait. Plus OpenBSD does not offer an iowait value
as Linux and thus the corresponding function would not be portable.

slstatus.h

@@ -7,7 +7,6 @@ const char *battery_state(const char *);
 /* cpu */
 const char *cpu_freq(void);
 const char *cpu_perc(void);
-const char *cpu_iowait(void);
 
 /* datetime */
 const char *datetime(const char *);