Remove an older hack in hotspot for booting in legacy mode

[Mandy Chung]

Mark, Karen,

When running from jdk build/<os>-<arch>/bin/java (where it has the 
"classes" directory), in legacy mode, when I recompile the jdk classes 
without updating the system module library, the VM will load the older 
copy in the module library instead of the build/classes directory since 
the boot module is hardcoded in the VM default bootclasspath.

This fix will remove this older hack put in hotspot to hardcode the boot 
module in the bootclasspath for legacy mode running in a module image.  
It's replaced by a recent hack I put in the launcher to prepend all 
modules in the system module library in the bootclasspath.

The change only impacts legacy mode.  In the module mode,  the 
bootclasspath is set differently (line 870-907) to the value of the 
sun.java.launcher.module.boot property (that is set by the launcher).  
It continues to work as it is.

Webrev at:
  http://cr.openjdk.java.net/~mchung/jigsaw/hotspot-sjlmb/

Thanks
Mandy