Debugger: Fix context menu item text

As it is possible to delete more than one selected breakpoint it
should be indicated in the menu.

Change-Id: Iabf6d4313e65bb65d7e822b05f448f459a7ae3a5
Reviewed-by: hjk <hjk@theqtcompany.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>