Revert "BUG #15874: Fix texture blending to discard transparent pixels."

This reverts commit 2c3f90a2.

This fixes BUG #15993. Discarding fragments with 0 alpha ended up
discarding pixels even when blending was disabled. That was causing the
bug reported in 159993. BUG #15874 has since been fixed by
paraview/paraview!598, this fix is no longer needed for the issue it was
added to fix (albeit incorrectly).

(cherry picked from commit e0a8c7da)

Rendering/OpenGL2/glsl/vtkTextureObjectFS.glsl

@@ -26,8 +26,4 @@ uniform sampler2D source;
 void main(void)
 {
   gl_FragData[0] = texture2D(source,tcoordVC);
-  if (gl_FragData[0].a <= 0.0)
-    {
-    discard;
-    }
 }