Use status_ask_choice() for the export dialog

Remove the export bar and use a simple status_ask_choice() dialog
instead. This reduces code complexity and replaces another unnecessary
menu by the well-tested dialog feature that is used everywhere else.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -743,7 +743,6 @@ void io_check_file(char *, int *);
 int io_check_data_files(void);
 void io_startup_screen(int);
 void io_export_data(enum export_type);
-void io_export_bar(void);
 void io_import_data(enum import_type, const char *);
 struct io_file *io_log_init(void);
 void io_log_print(struct io_file *, int, const char *);