Cleanup: Remove compilation warning.

In MTLTexture it was checked that this was valid. What in that
case should always be true.

source/blender/gpu/metal/mtl_texture.mm

@@ -275,7 +275,7 @@ void gpu::MTLTexture::blit(id<MTLBlitCommandEncoder> blit_encoder,
                            uint depth)
 {
 
-  BLI_assert(this && dest);
+  BLI_assert(dest);
   BLI_assert(width > 0 && height > 0 && depth > 0);
   MTLSize src_size = MTLSizeMake(width, height, depth);
   MTLOrigin src_origin = MTLOriginMake(src_x_offset, src_y_offset, src_z_offset);