erofs-utils: build: drop stale liblzma path handling

liblzma now uses PKG_CHECK_MODULES() for path discovery, but a
leftover assignment still rewrote liblzma_LIBS to plain `-llzma`,
dropping pkg-config linker flags such as `-L${prefix}/lib`, which
breaks custom liblzma path discovery.

Remove the redundant logic and drop the obsolete INSTALL.md reference
to --with-liblzma-{incdir,libdir}.

Reported-by: Guo Xuenan <guoxuenan@huawei.com>
Fixes: 37ada1b449ae ("erofs-utils: support liblzma auto-detection")
Assisted-by: Codex:GPT-5.5
Signed-off-by: Yifan Zhao <zhaoyifan28@huawei.com>
Link: https://lore.kernel.org/r/20260527100558.980152-1-zhaoyifan28@huawei.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>