erofs: clean up z_erofs_load_full_lcluster()

Only four lcluster types here, remove redundant code.
No real logic changes.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240508123357.3266173-1-hsiangkao@linux.alibaba.com