build: Drop xorg-x11 as a build dependency

The history on this one is complicated.  The note to README was added in
commit 1f95747a4f ("Add openssl-dev and xorg-x11-dev to README") in 2007.

At the time, there was a vendered version of Qemu in xen.git with a local
modification using <X11/keysymdef.h> to access the monitor console over VNC.

The final reference to keysymdef.h was dropped in commit 85896a7c4d ("build:
add autoconf to replace custom checks in tools/check") in 2012.  The next
prior mention was in 2009 with commit a8ccb671c3 ("tools: fix x11 check")
noting that x11 was not a direct dependcy of Xen; it was transitive through
SDL for Qemu for source-based distros.

It appears there may have been other unspecified dependencies on xorg,
e.g. the use of lndir by unmodified_drivers which are no longer relevant
either.

These days its only the Debian based dockerfiles which install xorg-x11, and
Qemu builds fine in these and others without x11.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

README

@@ -52,7 +52,6 @@ provided by your OS distributor:
     * Development install of zlib (e.g., zlib-dev)
     * Development install of Python 2.7 or later (e.g., python-dev)
     * Development install of curses (e.g., libncurses-dev)
-    * Development install of x11 (e.g. xorg-x11-dev)
     * Development install of uuid (e.g. uuid-dev)
     * Development install of yajl (e.g. libyajl-dev)
     * Development install of libaio (e.g. libaio-dev) version 0.3.107 or