RFR (XS): 8166930: minor cleanups 1) remove reference to ZIP_ReadMappedEntry 2) checking of st_mode

[Calvin Cheung]

Hi,

Please review this small change for two minor cleanups as mentioned in 
the bug report.

bug: https://bugs.openjdk.java.net/browse/JDK-8166930

webrev: http://cr.openjdk.java.net/~ccheung/8166930/webrev.00/

Testing:
     - JPRT with testset_hotspot
     - all tests under hotspot/runtime on the 8 supported platforms

thanks,
Calvin