Remove old files from SDL external

They were already removed in SDL upstream but the update mechanism did
not remove them in this repository.

They were causing build failures when the SDL external was used.

externals/SDL/src/render/vitagxm/shader_src/texture_tint_f.cg

@@ -1,4 +0,0 @@
-float4 main( float2 vTexcoord : TEXCOORD0, uniform sampler2D tex, uniform float4 uTintColor)
-{
-    return tex2D(tex, vTexcoord) * uTintColor;
-}