x86: remove unused thread arch member

The PDPT was moved to the stack area since it has alignment
requirements, but never removed from here.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

arch/x86/include/ia32/kernel_arch_thread.h

@@ -231,24 +231,6 @@ struct _thread_arch {
 	 * struct without ill effect.
 	 */
 	tPreempFloatReg preempFloatReg; /* volatile float register storage */
-
-#ifdef CONFIG_USERSPACE
-	/* Per-thread page directory pointer table when a thread is running
-	 * in user mode.
-	 *
-	 * With KPTI enabled, any privilege elevation while that thread is
-	 * running, or ISR will switch to the master kernel pdpt at
-	 * z_x86_kernel_pdpt; the memory domain policy will not apply at
-	 * all.
-	 *
-	 * With KPTI disabled, this pdpt will be active at all times when
-	 * the thread is running. This has implications for memory domain
-	 * partitions that are read-only!!
-	 *
-	 * See #17833 for more discussion.
-	 */
-	__aligned(0x20) struct x86_mmu_pdpt user_pdpt;
-#endif /* CONFIG_USERSPACE */
 };
 
 typedef struct _thread_arch _thread_arch_t;