When building with W=1:

    arch/m68k/apollo/config.c:111:5: warning: no previous prototype for ‘dn_serial_console_wait_key’ [-Wmissing-prototypes]
      111 | int dn_serial_console_wait_key(struct console *co) {
	  |     ^~~~~~~~~~~~~~~~~~~~~~~~~~
    arch/m68k/apollo/config.c:118:6: warning: no previous prototype for ‘dn_serial_console_write’ [-Wmissing-prototypes]
      118 | void dn_serial_console_write (struct console *co, const char *str,unsigned int count)
	  |      ^~~~~~~~~~~~~~~~~~~~~~~
    arch/m68k/apollo/config.c:238:6: warning: no previous prototype for ‘dn_dummy_waitbut’ [-Wmissing-prototypes]
      240 | void dn_dummy_waitbut(void) {
	  |      ^~~~~~~~~~~~~~~~

Fix this by removing these unused functions.
The last user of dn_serial_console_write() was removed in v2.3.41.
The other functions never had any users.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/5da9036914a5c84c20b6af55f1801f0c734a96bb.1694613528.git.geert@linux-m68k.org