sstate-cache-management.sh: fix remove duplicate failed when multiple archs

sstate-cache-management.sh: fix remove duplicate failed when multiple archs

The implementation of `--remove-duplicated' has been modified by the commit:
http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=0740f82aea92da0195524e37c372f9981e7f5e6d

In above commit, sstate cache files with multiple archs are not considered
duplicate and don't need to be removed as duplicated any more.

Update the description of `--remove-duplicated' in usage to keep consistent
with implementation.

[YOCTO #3635]

(From OE-Core rev: c201fdc2f01f398060cd953a1640a685797d9e64)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>