For erofs_map_blocks() and erofs_map_blocks_flatmode(), the flags
argument is always EROFS_GET_BLOCKS_RAW. Thus remove the unused flags
parameter for these two functions.
Besides EROFS_GET_BLOCKS_RAW is originally introduced for reading
compressed (raw) data for compressed files. However it's never used
actually and let's remove it now.
Signed-off-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx>