Remove NEED_PTHREAD_INIT macro and its usage in AIX hints

pthread_init is not a real function (in POSIX or AIX pthread), and
doesn't appear to have ever existed. Trying to use it causes a compiler
failure when using ibm-clang.

Author: FractalBoy

hints/aix_3.sh

@@ -229,7 +229,6 @@ case "$usethreads" in
 	d_setpwent_r='undef'
 	d_srand48_r='undef'
 	d_strerror_r='undef'
-	ccflags="$ccflags -DNEED_PTHREAD_INIT"
 	case "$cc" in
 	    *gcc*)
 		ccflags="-D_THREAD_SAFE $ccflags"

hints/aix_4.sh

@@ -315,7 +315,6 @@ case "$usethreads" in
 	d_srand48_r='undef'
 	d_strerror_r='undef'
 
-	ccflags="$ccflags -DNEED_PTHREAD_INIT"
 	case "$cc" in
 	    *gcc*)
 		ccflags="-D_THREAD_SAFE $ccflags"

thread.h

@@ -436,12 +436,6 @@ extern PERL_THREAD_LOCAL void *PL_current_context;
 #  endif /* PERL_SET_CONTEXT */
 #endif /* PERL_THREAD_LOCAL */
 
-#ifndef INIT_THREADS
-#  ifdef NEED_PTHREAD_INIT
-#    define INIT_THREADS pthread_init()
-#  endif
-#endif
-
 #ifndef ALLOC_THREAD_KEY
 #  define ALLOC_THREAD_KEY \
     STMT_START {						\