adapt blitbuffer API test to deletion of RGB16 format
adapt blitbuffer API test to deletion of RGB16 format

RGB16 was dropped since it won't likely be used. Reflect this by
removing the according test routine.

frankyifei/koreader-base,Frenzie/koreader-base,Hzj-jie/koreader-base,frankyifei/koreader-base,koreader/koreader-base,Frenzie/koreader-base,apletnev/koreader-base,NiLuJe/koreader-base,houqp/koreader-base,frankyifei/koreader-base,houqp/koreader-base,houqp/koreader-base,Frenzie/koreader-base,apletnev/koreader-base,koreader/koreader-base,houqp/koreader-base,koreader/koreader-base,NiLuJe/koreader-base,apletnev/koreader-base,frankyifei/koreader-base,Hzj-jie/koreader-base,NiLuJe/koreader-base,apletnev/koreader-base,NiLuJe/koreader-base,koreader/koreader-base,Hzj-jie/koreader-base,Hzj-jie/koreader-base,Frenzie/koreader-base