meson: remove incorrect and unneeded check for ptrdiff_t

Need to include stddef.h for it, so this would've worked:
if cc.has_type('ptrdiff_t', prefix: '#include <stddef.h>')

config.h.meson

@@ -229,9 +229,6 @@
 /* Have PTHREAD_PRIO_INHERIT. */
 #mesondefine HAVE_PTHREAD_PRIO_INHERIT
 
-/* Define to 1 if the system has the type `ptrdiff_t'. */
-#mesondefine HAVE_PTRDIFF_T
-
 /* Define if RDTSC is available */
 #mesondefine HAVE_RDTSC
 

meson.build

@@ -183,10 +183,6 @@ if cc.has_function('clock_gettime', prefix : '#include <time.h>')
   cdata.set('HAVE_CLOCK_GETTIME', 1)
 endif
 
-if cc.has_type('ptrdiff_t')
-  cdata.set('HAVE_PTRDIFF_T')
-endif
-
 # We only want to use the __declspec(dllexport/import) dance in GST_EXPORT when
 # building with MSVC
 if cc.get_id() == 'msvc'