gdb/doc: Remove mention of vCont's default actions

gdb/doc: Remove mention of vCont's default actions

Discussion with qemu folks suggests that the vCont description could
be even simpler and clearer.  Given we now say:

 For each inferior thread, the leftmost action with a matching
 thread-id is applied.

There's really no need to even talk about "default" actions, which
raises doubts about whether "default" is special in some way (it's
not).

See <https://lists.gnu.org/archive/html/qemu-devel/2016-10/msg06944.html>.

gdb/doc/ChangeLog:
2016-10-29  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Packets) <vCont>: Remove mention of default
	actions.