BLXX-194 / MXGDA-1538 Remove MOZILLA_FIVE_HOME from gdaclient

On Red Hat 5, setting this to the Firefox 8 install causes GDA to crash.
Without it being set Eclipse will just fall back to using an external browser.

On Red Hat 6 (at least some workstations, anyway), WebKit is installed and
Eclipse will prefer WebKit over Firefox; hence MOZILLA_FIVE_HOME is probably
irrelevant on RH 6 machines.