Remove documention of dead "target vxworks"

Remove documention of dead "target vxworks"

"target vxworks" and friends have been removed 10 years ago already:

  commit e84ecc995d6a5e4e9114d3cea61717b8a573afb6
  Author:     Andrew Cagney <cagney@redhat.com>
  AuthorDate: Sat Nov 13 23:10:02 2004 +0000

     2004-11-13  Andrew Cagney  <cagney@gnu.org>

         * configure.tgt: Delete i[34567]86-*-vxworks*, m68*-netx-*,
         m68*-*-vxworks*, mips*-*-vxworks*, powerpc-*-vxworks*, and
         sparc-*-vxworks*.
         * NEWS: Mention that vxworks was deleted.
     (...)
         * remote-vxmips.c, remote-vx.c: Delete.
         * remote-vx68.c: Delete.
     (...)

This removes related leftover cruft from the manual.

gdb/doc/
2014-09-16  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Starting) <run command>: Don't mention VxWorks.
	(Embedded OS): Remove VxWorks menu entry.
	(VxWorks): Remove node.