'cpoptions': remove "k" flag

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

src/nvim/api/vim.c

@@ -153,7 +153,6 @@ String nvim_replace_termcodes(String str, Boolean from_part, Boolean do_lt,
   char *ptr = NULL;
   // Set 'cpoptions' the way we want it.
   //    FLAG_CPO_BSLASH  set - backslashes are *not* treated specially
-  //    FLAG_CPO_KEYCODE set - keycodes are *not* reverse-engineered
   //    FLAG_CPO_SPECI unset - <Key> sequences *are* interpreted
   //  The third from end parameter of replace_termcodes() is true so that the
   //  <lt> sequence is recognised - needed for a real backslash.

src/nvim/option_defs.h

@@ -97,7 +97,6 @@
 #define CPO_INTMOD      'i'     /* interrupt a read makes buffer modified */
 #define CPO_INDENT      'I'     /* remove auto-indent more often */
 #define CPO_ENDOFSENT   'J'     /* need two spaces to detect end of sentence */
-#define CPO_KEYCODE     'k'     /* don't recognize raw key code in mappings */
 #define CPO_KOFFSET     'K'     /* don't wait for key code in mappings */
 #define CPO_LITERAL     'l'     /* take char after backslash in [] literal */
 #define CPO_LISTWM      'L'     /* 'list' changes wrapmargin */
@@ -132,7 +131,7 @@
 #define CPO_CHANGEW     '_'     // "cw" special-case
 // default values for Vim and Vi
 #define CPO_VIM         "aABceFs_"
-#define CPO_VI          "aAbBcCdDeEfFiIJkKlLmMnoOpPqrRsStuvWxXyZ$!%+<>;_"
+#define CPO_VI          "aAbBcCdDeEfFiIJKlLmMnoOpPqrRsStuvWxXyZ$!%+<>;_"
 
 /* characters for p_ww option: */
 #define WW_ALL          "bshl<>[],~"