refactor: remove CPO_TO_CPO_FLAGS() (#26718)

Just pass p_cpo to replace_termcodes() directly.
This allows removing option_vars.h from keycodes.h, and also avoids the
mistake of passing 0 as cpo_flags.

src/nvim/keycodes.h

@@ -4,7 +4,6 @@
 
 #include "nvim/ascii_defs.h"
 #include "nvim/eval/typval_defs.h"  // IWYU pragma: keep
-#include "nvim/option_vars.h"
 
 // Keycode definitions for special keys.
 //
@@ -475,11 +474,6 @@ enum key_extra {
 /// This is a total of 6 tokens, and is currently the longest one possible.
 #define MAX_KEY_CODE_LEN    6
 
-#define FLAG_CPO_BSLASH    0x01
-#define CPO_TO_CPO_FLAGS   ((vim_strchr((char *)p_cpo, CPO_BSLASH) == NULL) \
-                            ? 0 \
-                            : FLAG_CPO_BSLASH)
-
 /// Flags for replace_termcodes()
 enum {
   REPTERM_FROM_PART   = 1,