Fix T81002: Images drawn with the Python gpu module no longer draw on top in the Image Editor

Since {D8234} the image editor is drawn using a depth buffer.
When using `draw_texture_2d` the image is drawn using the 2D_IMAGE
shader. inside the vertex buffer the image was pushed to the background.
This was introduced by {648924333234} what seems to be out dated as we
have done several overhauls in this area. (workbench refactor, overlay
engine refactor, color management pipeline).

This patch removes the pushing of the image to the background.

source/blender/gpu/shaders/gpu_shader_2D_image_vert.glsl

@@ -9,6 +9,5 @@ out vec2 texCoord_interp;
 void main()
 {
   gl_Position = ModelViewProjectionMatrix * vec4(pos.xy, 0.0f, 1.0f);
-  gl_Position.z = 1.0;
   texCoord_interp = texCoord;
 }