The Wikipadia entry on the Harrier Gr 5 et al is, of course, ........ However, the entry on the 25mm Aden cannon developed for the GR 5 is more reliable:
http://en.wikipedia.org/wiki/ADEN_cannon
Basivcally, it got scrubbed in 1999 and was never replaced. The pods remained in situ as the a/c gains quite a bit of "lift" from the circulating air between the pods when going VTOL, which is why the pods were never removed. I think they house some ECM gear now.
Here is the best discussion thread I could find on the subject, from the Air Forces Monthly Forum:
http://forum.keypublishing.co.uk/showthread.php?t=62971
The most cogent posting is No. 24 by a chap called Tony Williams.
I would accept postings by him as pretty spot-on:
http://www.quarry.nildram.co.uk/miltech.htm
Remember, the Sea Harrier still had a gun until the aircraft's retirement in March 2006
Hope this helps.