x86/proresdsp: remove unused macro

Reviewed-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/x86/proresdsp.asm

@@ -300,20 +300,6 @@ cglobal prores_idct_put_10, 4, 4, %1
     RET
 %endmacro
 
-%macro SIGNEXTEND 2-3
-%if cpuflag(sse4) ; dstlow, dsthigh
-    movhlps     %2,  %1
-    pmovsxwd    %1,  %1
-    pmovsxwd    %2,  %2
-%elif cpuflag(sse2) ; dstlow, dsthigh, tmp
-    pxor        %3,  %3
-    pcmpgtw     %3,  %1
-    mova        %2,  %1
-    punpcklwd   %1,  %3
-    punpckhwd   %2,  %3
-%endif
-%endmacro
-
 INIT_XMM sse2
 idct_put_fn 16
 INIT_XMM sse4