memline: do not attempt to cache line2byte()

This is irrelevant to neovim usecases anyway.

src/nvim/memline_defs.h

@@ -58,6 +58,7 @@ typedef struct memline {
   linenr_T ml_line_lnum;        // line number of cached line, 0 if not valid
   char_u      *ml_line_ptr;     // pointer to cached line
   size_t ml_line_offset;        // cached byte offset of ml_line_lnum
+  int ml_line_offset_ff;        // fileformat of cached line
 
   bhdr_T      *ml_locked;       // block used by last ml_get
   linenr_T ml_locked_low;       // first line in ml_locked