Fix: wrong member name WebGLCubeUVMaps.onTextureDispose

[OndrejSpanel]

The member name used in WebGLCubeUVMaps.onTextureDispose is wrong. It should be cubeUVmaps.delete(texture), not cubemapUV.delete(texture), which calls who knows what (if anything at all).

This is also confirmed by looking at similar WebGLCubeMaps.onTextureDispose:

			cubemaps.delete( texture );
			cubemap.dispose();

[mrdoob]

Thanks!