Remove bitwise plane information from ImageFormat's P010 docs

Some vendors may prefer a different underlying layout for P010 in the
future compared to the one currently supported in gralloc. In
particular, it's not a guarantee that the CB/CR plane is contiguous in
memory with the Y plane.

Bug: 195570363
Test: builds
Change-Id: I01eea78b8555610470959cdb4f0f9ceb16e80f73