Removed the duplicated function body in the Cortex-M4 AC6 port (#589)

_tx_thread_system_return_inline() in the Cortex-M4 AC6 tx_port.h was followed
by a second, orphaned copy of its own body. The copy had no function header, so
it declared interrupt_save at file scope and then placed statements there,
which does not compile. It is also the older version of the body, without the
dsb and isb barriers, so it was left behind rather than intended: the barriers
were added by commit 33efad3f and the previous text was not removed.

Delete the orphaned copy. What remains is the same body every sibling port
carries: after this change ports/cortex_m4/ac6/inc/tx_port.h differs from
ports/cortex_m4/iar/inc/tx_port.h and ports/cortex_m7/ac6/inc/tx_port.h only in
the port name in the banner and the version string, as it should.

Verified by compiling the function in isolation, which fails on the file scope
statements before the change and is clean afterwards, and by a structural scan
of all 208 tx_port.h files in the repository confirming this was the only
occurrence.

Fixes https://github.com/eclipse-threadx/threadx/issues/569

Assisted-by: Claude Code (Opus 5) <noreply@anthropic.com>

ports/cortex_m4/ac6/inc/tx_port.h

@@ -11,6 +11,7 @@
 
 // Some portions generated by Gemini (Gemini 2.0 Flash).
 // Some portions generated by Copilot (Claude Sonnet 4.6).
+// Some portions generated by Claude Code (Opus 5).
 
 
 /**************************************************************************/
@@ -594,21 +595,6 @@ unsigned int interrupt_save;
         __restore_interrupt(interrupt_save);
     }
 }
-unsigned int interrupt_save;
-
-    /* Set PendSV to invoke ThreadX scheduler.  */
-    *((volatile ULONG *) 0xE000ED04) = ((ULONG) 0x10000000);
-    if (__get_ipsr_value() == 0)
-    {
-        interrupt_save = __get_interrupt_posture();
-#ifdef TX_PORT_USE_BASEPRI
-        __set_basepri_value(0);
-#else
-        __enable_interrupts();
-#endif
-        __restore_interrupt(interrupt_save);
-    }
-}
 
 #define TX_INTERRUPT_SAVE_AREA                  UINT interrupt_save;
 #define TX_DISABLE                              interrupt_save =  __disable_interrupts();