MDEV-17750: Fix the Windows build

Remove some redundant declarations of global variables. These declarations
can refer to undefined type names, because they occur before any #include
statement for the type definitions. On GNU/Linux, uint and ulong seem to
be defined in <sys/types.h> due to _DEFAULT_SOURCE and __USE_MISC
becoming defined in <features.h>.

storage/innobase/include/ut0mutex.h

@@ -29,9 +29,6 @@ Created 2012-03-24 Sunny Bains.
 #ifndef ut0mutex_h
 #define ut0mutex_h
 
-extern uint	srv_spin_wait_delay;
-extern ulong	srv_n_spin_wait_rounds;
-
 #include "sync0policy.h"
 #include "ib0mutex.h"
 #include <set>