* GPU: Remove CPU region handle containers.

Another one for the "I don't know why I didn't do this earlier" pile.

This removes the "Cpu" prefixed region handle classes, which each mirror a region handle type from Ryujinx.Memory.

Originally, not all projects had a reference to Ryujinx.Memory, so these classes were introduced to bridge the gap. Someone else crossed that bridge since, so these classes don't have much of a purpose anymore.

This PR replaces all uses of CpuRegionHandle etc to their direct Ryujinx.Memory versions.

RegionHandle methods (specifically QueryModified) are about the hottest path there is in the entire emulator, so there is a nice boost from doing this.

* Add docs