Merge pull request #21360 from Mugen87/dev51

WebGLBackground: Remove support for WebGLCubeRenderTarget.

src/renderers/webgl/WebGLBackground.js

@@ -58,7 +58,7 @@ function WebGLBackground( renderer, cubemaps, state, objects, premultipliedAlpha
 
 		}
 
-		if ( background && ( background.isCubeTexture || background.isWebGLCubeRenderTarget || background.mapping === CubeUVReflectionMapping ) ) {
+		if ( background && ( background.isCubeTexture || background.mapping === CubeUVReflectionMapping ) ) {
 
 			if ( boxMesh === undefined ) {
 
@@ -100,14 +100,6 @@ function WebGLBackground( renderer, cubemaps, state, objects, premultipliedAlpha
 
 			}
 
-			if ( background.isWebGLCubeRenderTarget ) {
-
-				// TODO Deprecate
-
-				background = background.texture;
-
-			}
-
 			boxMesh.material.uniforms.envMap.value = background;
 			boxMesh.material.uniforms.flipEnvMap.value = ( background.isCubeTexture && background._needsFlipEnvMap ) ? - 1 : 1;