Remove stray trailing "," from DAP breakpoint.py

The buildbot pointed out that the last DAP series I checked in had an
issue.  Looking into it, it seems there is a stray trailing "," in
breakpoint.py.  This patch removes it.

This seems to point out a test suite deficiency.  I will look into
fixing that.

gdb/python/lib/gdb/dap/breakpoint.py

@@ -119,7 +119,7 @@ def _breakpoint_descriptor(bp):
             )
 
         if loc.address:
-            result["instructionReference"] = hex(loc.address),
+            result["instructionReference"] = hex(loc.address)
 
     return result