sstate-cache-management.sh: fix it for downloaded file

sstate-cache-management.sh: fix it for downloaded file

Fix it for the downloaded sstate cache file
* The downloaded sstate cache file is in SSTATE_DIR, and there is a
  symlink points to it, the sstate cache file should not be removed if
  the corresponding symlink is kept, it would be removed when the
  symlink is removed (with -L option).

* Fix a comment line.

[YOCTO #2897]

(From OE-Core rev: 12ed7e6bb215e70b119d5ce123e36e2fd380ff1e)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>