ext4: remove unused EXT_STATS macro from ext4_extents.h

ext4: remove unused EXT_STATS macro from ext4_extents.h

The EXT_STATS macro in fs/ext4/ext4_extents.h has been defined
but never used in the codebase since its introduction. This patch
removes it.

Analysis:
1. No references found in fs/ext4/ or other kernel code.
2. No impact on compilation or functionality.
3. Git history shows it was never utilized.

Signed-off-by: Baolin Liu <liubaolin@kylinos.cn>
Reviewed-by: Baokun Li <libaokun1@huawei.com>
Link: https://patch.msgid.link/20250527053805.1550912-1-liubaolin12138@163.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>