Removing is_retrofit code path

This code path was never invoked. is_logical will return false on
secondary partitions in retrofit devices, so nothing actually is ever
deleted. If we manage to call the delete, the device side code will
fail with "cannot open the super partition"

Test: fastboot flashall on sargo device
Change-Id: I20b430c5c30bf992506190ea4e00b0b69c7b1005