input: disallow autorepeat by default for most commands

This drops autorepeated key events for a number of commands. This should
help with slow situations accidentally triggering too many repeats. (I'm
not sure if this actually happened to users - maybe.)

It's not clear whether MP_CMD_COMMAND_LIST (multiple commands on one
binding separated by ';') should be repeated, or whether it should try
to do something clever. For now, disallow autorepeat with it.
input: remove unused key_down_event command

There's no real use-case for this, and is wasn't documented (didn't even
appear on the "undocumented commands" list).