ums: remove UMS_{NUM,START}_SECTORS + UMS_START_SECTOR

These values aren't set anywhere at present, and hence have no effect.
The concept of a single global offset/number of sectors to expose through
USB Mass Storage doesn't even make sense in the face of multiple storage
devices. Remove these defines to simplify the code.

Cc: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Przemyslaw Marczak <p.marczak@samsung.com>