remove !has_mbyte branches

Ref: https://github.com/neovim/neovim/pull/10171#discussion_r292437832

src/nvim/change.c

@@ -503,7 +503,6 @@ void ins_bytes(char_u *p)
 /// Handles Replace mode and multi-byte characters.
 void ins_bytes_len(char_u *p, size_t len)
 {
-  if (has_mbyte) {
   size_t n;
   for (size_t i = 0; i < len; i += n) {
     if (enc_utf8) {
@@ -514,11 +513,6 @@ void ins_bytes_len(char_u *p, size_t len)
     }
     ins_char_bytes(p + i, n);
   }
-  } else {
-    for (size_t i = 0; i < len; i++) {
-      ins_char(p[i]);
-    }
-  }
 }
 
 /// Insert or replace a single character at the cursor position.
@@ -597,11 +591,7 @@ void ins_char_bytes(char_u *buf, size_t charlen)
     // first (it tells the byte length of the character).
     replace_push(NUL);
     for (size_t i = 0; i < oldlen; i++) {
-      if (has_mbyte) {
       i += (size_t)replace_push_mb(oldp + col + i) - 1;
-      } else {
-        replace_push(oldp[col + i]);
-      }
     }
   }
 
@@ -688,7 +678,6 @@ void ins_str(char_u *s)
 // return FAIL for failure, OK otherwise
 int del_char(bool fixpos)
 {
-  if (has_mbyte) {
   // Make sure the cursor is at the start of a character.
   mb_adjust_cursor();
   if (*get_cursor_pos_ptr() == NUL) {
@@ -696,8 +685,6 @@ int del_char(bool fixpos)
   }
   return del_chars(1L, fixpos);
 }
-  return del_bytes(1, fixpos, true);
-}
 
 /// Like del_bytes(), but delete characters instead of bytes.
 int del_chars(long count, int fixpos)
@@ -983,11 +970,7 @@ int open_line(
     replace_push(NUL);
     p = saved_line + curwin->w_cursor.col;
     while (*p != NUL) {
-      if (has_mbyte) {
       p += replace_push_mb(p);
-      } else {
-          replace_push(*p++);
-      }
     }
     saved_line[curwin->w_cursor.col] = NUL;
   }