Remove support for testing against dead "target vxworks"

Remove support for testing against 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 testsuite.

Tested on x86_64 Fedora 20, native and gdbserver.

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

	* config/vx.exp, config/vxworks.exp, config/vxworks29k.exp: Delete
	files.
	* gdb.base/a2-run.exp: Remove all code guarded by istarget
	"*-*-vxworks*" throughout.
	* gdb.base/break.exp: Likewise.
	* gdb.base/default.exp: Likewise.
	* gdb.base/scope.exp: Likewise.
	* gdb.base/sepdebug.exp: Likewise.
	* gdb.base/break.c: Remove all code guarded by #ifdef vxworks
	throughout.
	* gdb.base/run.c: Likewise.
	* gdb.base/sepdebug.c: Likewise.
	* gdb.hp/gdb.aCC/run.c: Likewise.
	* gdb.reverse/until-reverse.c: Likewise.
	* lib/gdb.exp (gdb_compile): Remove is_vxworks branch.