Give up on the X11 frontend for now

There seem to be only a few things it could bring to the table,
compared to xP, making it barely worth the effort:

 - saner keyboard controls,
 - GVIM integration,
 - slightly improved resource usage.

README.adoc

@@ -2,7 +2,7 @@ xK
 ==
 
 'xK' (chat kit) is an IRC software suite consisting of a daemon, bot, terminal
-client, and X11/web frontends for the client.  It's all you're ever going to
+client, and a web frontend for the client.  It's all you're ever going to
 need for chatting, so long as you can make do with slightly minimalist software.
 
 They're all lean on dependencies, and offer a maximally permissive licence.
@@ -26,16 +26,9 @@ As a unique bonus, you can launch a full text editor from within.
 xP
 --
 The web frontend for 'xC', making use of its networked relay interface.
-Fully usable, despite some rough edges.  It intentionally differs in that it
-uses a sans-serif font, and shows the list of all buffers in a side panel.
-Otherwise it aims to be a near replica.
-
-xF
---
-The X11 frontend for 'xC', making use of its networked relay interface.
-
-This subproject has been put on hold, partly because of its massive overlap
-with 'xP', and is hidden behind a CMake option.
+It intentionally differs in that it uses a sans-serif font, and it shows
+the list of all buffers in a side panel.  Otherwise it is a near replica,
+including keyboard shortcuts.
 
 xD
 --
@@ -78,7 +71,6 @@ Build-only dependencies:
 Common runtime dependencies: openssl +
 Additionally for 'xC': curses, libffi, readline >= 6.0 or libedit >= 2013-07-12,
  lua >= 5.3 (optional) +
-Additionally for 'xF': x11, xft
 
  $ git clone --recursive https://git.janouch.name/p/xK.git
  $ mkdir xK/build