'cpoptions': remove "k" flag

This was already removed in 3baba1e7bc, except the documentation and
CPO_VI entry. find_term_bykeys() is irrelevant to Nvim.

runtime/doc/map.txt

@@ -444,8 +444,7 @@ There are two ways to map a special key:
    starts with <Esc>.  To enter a mapping like this you type ":map " and then
    you have to type CTRL-V before hitting the function key.  Note that when
    the key code for the key is in the |terminfo| entry, it will automatically
-   be translated into the internal code and become the second way of mapping
-   (unless the 'k' flag is included in 'cpoptions').
+   be translated into the internal code and become the second way of mapping.
 2. The second method is to use the internal code for the function key.  To
    enter such a mapping type CTRL-K and then hit the function key, or use
    the form "#1", "#2", .. "#9", "#0", "<Up>", "<S-Down>", "<S-F7>", etc.

runtime/doc/options.txt

@@ -1618,15 +1618,6 @@ A jump table for the options with a short description can be found at |Q_op|.
 		J	A |sentence| has to be followed by two spaces after
 			the '.', '!' or '?'.  A <Tab> is not recognized as
 			white space.
-								*cpo-k*
-		k	Disable the recognition of raw key codes in
-			mappings, abbreviations, and the "to" part of menu
-			commands.  For example, if <Key> sends ^[OA (where ^[
-			is <Esc>), the command ":map X ^[OA" results in X
-			being mapped to:
-				'k' included:	"^[OA"	 (3 characters)
-				'k' excluded:	"<Key>"  (one key code)
-			Also see the '<' flag below.
 								*cpo-K*
 		K	Don't wait for a key code to complete when it is
 			halfway through a mapping.  This breaks mapping

runtime/doc/vim_diff.txt

@@ -332,7 +332,7 @@ Test functions:
 
 Other options:
   'antialias'
-  'cpoptions' ("g", "w", "H", "*", "-", "j", and all POSIX flags were removed)
+  'cpoptions' (g j k H w * - and all POSIX flags were removed)
   'encoding' ("utf-8" is always used)
   'esckeys'
   'guioptions' "t" flag was removed