erofs: remove a superfluous check for encoded extents

It is possible when an inode is split into segments for multi-threaded
compression, and the tail extent of a segment could also be small.

Fixes: 1d191b4c ("erofs: implement encoded extent metadata")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250620153108.1368029-1-hsiangkao@linux.alibaba.com