gnu: racket: Update to 8.5.

gnu: racket: Update to 8.5.

* gnu/packages/patches/racket-gui-tethered-launcher-backport.patch,
gnu/packages/patches/racket-enable-scheme-backport.patch: Delete files.
* gnu/local.mk (dist_patch_DATA): Remove them.
* gnu/packages/chez.scm (chez-scheme-for-racket): Update to 9.5.7.6.
* gnu/packages/racket.scm (%racket-version): Update to 8.5.
(%racket-origin)[patches]: Remove obsolete patches.
(racket-vm-common-configure-flags): Correct comment.
(racket)[inputs]<2d, datalog, deinprogramm, draw, drracket, expeditor,
htdp, pict, plot, quickscript, racklog, rackunit, redex, scribble,
typed-racket, string-constants, syntax-color, web-server, xrepl>: Update
checksums.
<gui, srfi>: Likewise, and remove obsolete backports.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>