Remove display_erros()

do_shell() in Nvim uses the Nvim UI, not the tty directly, so
display_errors() is not necessary anymore.

src/nvim/ex_cmds.c

@@ -1391,9 +1391,6 @@ do_shell(
   msg_row = Rows - 1;
   msg_col = 0;
 
-  // display any error messages now
-  display_errors();
-
   apply_autocmds(EVENT_SHELLCMDPOST, NULL, NULL, FALSE, curbuf);
 }
 

src/nvim/vim.h

@@ -291,7 +291,6 @@ enum { FOLD_TEXT_LEN = 51 };  //!< buffer size for get_foldtext()
 # define mch_errmsg(str)        fprintf(stderr, "%s", (str))
 # define mch_msg(str)           printf("%s", (str))
 #endif
-#define display_errors()       fflush(stderr)
 
 #include "nvim/globals.h"        // global variables and messages
 #include "nvim/buffer_defs.h"    // buffer and windows