Remove the documentation for the <s-x> keybinding

It was removed in commit 0e1632868e and it
is now the standard kill-region command.

README.md

@@ -310,7 +310,6 @@ Keybinding         | Description
 <kbd>C-c . %</kbd> | Replace integer at point with another specified integer.
 <kbd>C-c . '</kbd> | Perform arithmetic operations on integer at point. User specifies the operator.
 <kbd>Super-r</kbd> | Recent files
-<kbd>Super-x</kbd> | Expand region
 <kbd>Super-j</kbd> | Join lines
 <kbd>Super-k</kbd> | Kill whole line
 <kbd>Super-m m</kbd> | Magit status

core/prelude-core.el

@@ -382,7 +382,7 @@ Doesn't mess with special buffers."
     "Press <C-c p f> or <s-f> to navigate a project's files with ido."
     "Press <C-c p g> or <s-g> to run grep on a project."
     "Press <C-c p s> or <s-p> to switch between projects."
-    "Press <C-=> or <s-x> to expand the selected region."
+    "Press <C-=> to expand the selected region."
     "Press <C-c g> to search in Google."
     "Press <C-c G> to search in GitHub."
     "Press <C-c y> to search in YouTube."