thread.c: remove vestigial CONFIG_INIT_STACKS cruft

It looks like, at some point in the past, initializing thread stacks
was the responsibility of the arch layer. After that was centralized,
we forgot to remove the related conditional header inclusion. Fixed.

Signed-off-by: Charles E. Youse <charles.youse@intel.com>
diff --git a/arch/arc/core/thread.c b/arch/arc/core/thread.c
index ec402ca..137ce05 100644
--- a/arch/arc/core/thread.c
+++ b/arch/arc/core/thread.c
@@ -16,9 +16,6 @@
 #include <kernel_structs.h>
 #include <offsets_short.h>
 #include <wait_q.h>
-#ifdef CONFIG_INIT_STACKS
-#include <string.h>
-#endif /* CONFIG_INIT_STACKS */
 
 #ifdef CONFIG_USERSPACE
 #include <arch/arc/v2/mpu/arc_core_mpu.h>
diff --git a/arch/posix/core/thread.c b/arch/posix/core/thread.c
index e99f636..bfe1368 100644
--- a/arch/posix/core/thread.c
+++ b/arch/posix/core/thread.c
@@ -13,10 +13,6 @@
  * architecture
  */
 
-#ifdef CONFIG_INIT_STACKS
-#include <string.h>
-#endif /* CONFIG_INIT_STACKS */
-
 #include <toolchain.h>
 #include <kernel_structs.h>
 #include <wait_q.h>
diff --git a/arch/x86/core/ia32/thread.c b/arch/x86/core/ia32/thread.c
index d5ea897..68563be 100644
--- a/arch/x86/core/ia32/thread.c
+++ b/arch/x86/core/ia32/thread.c
@@ -12,10 +12,6 @@
  * processor architecture.
  */
 
-#ifdef CONFIG_INIT_STACKS
-#include <string.h>
-#endif /* CONFIG_INIT_STACKS */
-
 #include <toolchain.h>
 #include <linker/sections.h>
 #include <kernel_structs.h>
diff --git a/arch/xtensa/core/thread.c b/arch/xtensa/core/thread.c
index 06ee316..5a7a307 100644
--- a/arch/xtensa/core/thread.c
+++ b/arch/xtensa/core/thread.c
@@ -3,9 +3,6 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
-#ifdef CONFIG_INIT_STACKS
-#include <string.h>
-#endif /* CONFIG_INIT_STACKS */
 #ifdef CONFIG_DEBUG
 #include <sys/printk.h>
 #endif