Retire cpu_count()

Set threads number always to 1 at startup and let the
user explicitly to chose the number of threads.

Also preserve the useful behavior of automatically set
"Min Split Depth" according to the requested threads,
indeed this parameter is too technical for a casual user,
so, when left to zero, we set it on a sensible value.

No functional change

src/misc.h

@@ -27,7 +27,6 @@
 #include "types.h"
 
 extern const std::string engine_info(bool to_uci = false);
-extern int cpu_count();
 extern void timed_wait(WaitCondition&, Lock&, int);
 extern void prefetch(char* addr);
 extern void start_logger(bool b);