Remove i.MX5 and i.MX6 workaround to avoid mesa-demos inclusion

mesa-demos was fixed and now it can be built for non-mesa
implementations as well. So, it does not need to be artificially
removed from images.

Change-Id: I2824aa478161dd2d04ec9f386978a4e2c31e7093
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>