Update arcsde plugin to work with the gt-arcsde-2.6-SNAPSHOT

uDig's arcsde is not going to work correctly with the latest geotools arcsde module:

• remove the shipped esri jars, they're 9.0 and won't work with geotools (9.0 support has officially dropped on geotools though it should still work provided the esri jars are 9.2+)
• force the user to indicate the location of the 3 required jars
• clean up only: remove references in code to postgis and jdbc. They're left there as a result of the module being created as a copy of the postgis one I concur, but arcsde has nothing to do with neither postgis nor jdbc