Is this going to work like this forever
After further analysis, I think this a definte case of the "Compute Build Path" function being overly agressive (and simplistic). This is really a problem with the PDE itself that has nothing specific to do with SWT Designer. I tried some tests involving other fragment jars added by other plugins (both open source and commercial) and the same thing happened. The fragment jars were all pulled in even though none of them were needed. The PDE should either never include fragment jars in that computation, or it should have an option to ignore them.
On the bright side, there is a very simple work around. All you need to do is delete that line from the .classpath file and you will be fine. That jar file is not needed
by your plugin in any way. Personally, I don't even like using the "Compute Build Path" function as it tends to screw up other aspects of the .classpath file.