armv8: Don't touch the x18 register

The x18 register is platform specific, and using it in portable assembler
code should be avoided.

In this particular case, using it is not actually needed at all.

Change-Id: I07016fdd3a264ed77eba37abcd1748b51cf7a49e
diff --git a/encoder/armv8/ih264e_evaluate_intra16x16_modes_av8.s b/encoder/armv8/ih264e_evaluate_intra16x16_modes_av8.s
index e24ef49..df06d41 100644
--- a/encoder/armv8/ih264e_evaluate_intra16x16_modes_av8.s
+++ b/encoder/armv8/ih264e_evaluate_intra16x16_modes_av8.s
@@ -96,7 +96,6 @@
 
     ldr       x16, [sp, #80]
     mov       x17, x4
-    mov       x18, x5
     mov       x14, x6
     mov       x15, x7