Canvas renderer: Remove redundant assignment

[stuartcarnie]

Removes a redundant assignment that could result in bugs if reordered.

The original assignment is here:

godot/servers/rendering/renderer_rd/renderer_canvas_render_rd.cpp

Line 1938 in 5b5dc00

shader.quad_index_array = RD::get_singleton()->index_array_create(shader.quad_index_buffer, 0, 6);

and then overwritten by the following lines:

godot/servers/rendering/renderer_rd/renderer_canvas_render_rd.cpp

Lines 1941 to 1946 in 99a7a9c

	{ //primitive
	primitive_arrays.index_array[0] = shader.quad_index_array = RD::get_singleton()->index_array_create(shader.quad_index_buffer, 0, 1);
	primitive_arrays.index_array[1] = shader.quad_index_array = RD::get_singleton()->index_array_create(shader.quad_index_buffer, 0, 2);
	primitive_arrays.index_array[2] = shader.quad_index_array = RD::get_singleton()->index_array_create(shader.quad_index_buffer, 0, 3);
	primitive_arrays.index_array[3] = shader.quad_index_array = RD::get_singleton()->index_array_create(shader.quad_index_buffer, 0, 6);
	}

[akien-mga]

Thanks!