MDEV-17200 - pthread_detach called for already detached threads

pthread_detach_this_thread() was intended to be defined to something
meaningful only on some ancient unixes, which don't have
pthread_attr_setdetachstate() defined. Otherwise, on normal unixes,
threads are created detached in the first place.

This was broken in 0f01bf26 so that
we started calling pthread_detach() for already detached threads.
Intention was to detach aria checkpoint thread.

However in 87007dc2 aria service threads
were made joinable with appropriate handling, which makes breaking
revision unneccessary.

Revert remnants of 0f01bf26, so that
pthread_detach_this_thread() is meaningful only on some ancient unixes
again.

include/my_pthread.h

@@ -184,7 +184,7 @@ int pthread_cancel(pthread_t thread);
 #define pthread_key(T,V) pthread_key_t V
 #define my_pthread_getspecific_ptr(T,V) my_pthread_getspecific(T,(V))
 #define my_pthread_setspecific_ptr(T,V) pthread_setspecific(T,(void*) (V))
-#define pthread_detach_this_thread() { pthread_t tmp=pthread_self() ; pthread_detach(tmp); }
+#define pthread_detach_this_thread()
 #define pthread_handler_t EXTERNC void *
 typedef void *(* pthread_handler)(void *);