undo: delete undo_off global without effect

src/nvim/getchar.c

@@ -2409,7 +2409,6 @@ int inchar(
     did_outofmem_msg = FALSE;       /* display out of memory message (again) */
     did_swapwrite_msg = FALSE;      /* display swap file write error again */
   }
-  undo_off = FALSE;                 /* restart undo now */
 
   // Get a character from a script file if there is one.
   // If interrupted: Stop reading script files, close them all.

src/nvim/globals.h

@@ -768,7 +768,6 @@ EXTERN int did_outofmem_msg INIT(= false);
 // set after out of memory msg
 EXTERN int did_swapwrite_msg INIT(= false);
 // set after swap write error msg
-EXTERN int undo_off INIT(= false);          // undo switched off for now
 EXTERN int global_busy INIT(= 0);           // set when :global is executing
 EXTERN int listcmd_busy INIT(= false);      // set when :argdo, :windo or
                                             // :bufdo is executing

src/nvim/undo.c

@@ -225,9 +225,6 @@ int u_save_cursor(void)
  */
 int u_save(linenr_T top, linenr_T bot)
 {
-  if (undo_off)
-    return OK;
-
   if (top >= bot || bot > (curbuf->b_ml.ml_line_count + 1)) {
     return FAIL;        /* rely on caller to do error messages */
   }
@@ -246,9 +243,6 @@ int u_save(linenr_T top, linenr_T bot)
  */
 int u_savesub(linenr_T lnum)
 {
-  if (undo_off)
-    return OK;
-
   return u_savecommon(lnum - 1, lnum + 1, lnum + 1, FALSE);
 }
 
@@ -260,9 +254,6 @@ int u_savesub(linenr_T lnum)
  */
 int u_inssub(linenr_T lnum)
 {
-  if (undo_off)
-    return OK;
-
   return u_savecommon(lnum - 1, lnum, lnum + 1, FALSE);
 }
 
@@ -275,9 +266,6 @@ int u_inssub(linenr_T lnum)
  */
 int u_savedel(linenr_T lnum, long nlines)
 {
-  if (undo_off)
-    return OK;
-
   return u_savecommon(lnum - 1, lnum + nlines,
       nlines == curbuf->b_ml.ml_line_count ? 2 : lnum, FALSE);
 }
@@ -2925,9 +2913,6 @@ void u_undoline(void)
   colnr_T t;
   char_u  *oldp;
 
-  if (undo_off)
-    return;
-
   if (curbuf->b_u_line_ptr == NULL
       || curbuf->b_u_line_lnum > curbuf->b_ml.ml_line_count) {
     beep_flush();