Simplify the MetalTexture abstraction (AcademySoftwareFoundation#2095)

`mx_texture.metal` is used to provide an abstraction to the texture code when being called from GLSL code.

There are a few functions in the interface of this abstraction that do not appear to be used anywhere.

This PR proposes removing them, as a precursor to other upcoming texture work.  I'm proposing this as a separate PR to make things easier to review.

All of the MSL unit tests pass.

libraries/stdlib/genmsl/lib/mx_texture.metal

@@ -2,13 +2,8 @@ struct MetalTexture
 {
     texture2d<float> tex;
     sampler s;
-    int get_width() { return tex.get_width(); }
-    int get_height() { return tex.get_height(); }
-    int get_num_mip_levels() { return tex.get_num_mip_levels(); }
 };
 
-int get_width(MetalTexture mtlTex) { return mtlTex.get_width(); }
-
 float4 texture(MetalTexture mtlTex, float2 uv)
 {
     return mtlTex.tex.sample(mtlTex.s, uv);
@@ -21,10 +16,5 @@ float4 textureLod(MetalTexture mtlTex, float2 uv, float lod)
 
 int2 textureSize(MetalTexture mtlTex, int mipLevel)
 {
-    return int2(mtlTex.get_width(), mtlTex.get_height());
-}
-
-int texture_mips(MetalTexture mtlTex)
-{
-    return mtlTex.tex.get_num_mip_levels();
+    return int2(mtlTex.tex.get_width(), mtlTex.tex.get_height());
 }