nixosTests.keymap: Remove unnecessary sleep

When test-input-reader runs, it's standard input exists and will
be buffered, so by the time the file exists, the standard input
can already be written to.
I have no reason to believe that a terminal emulator would start
accepting input _after_ launching the command.
I've tested this for hours in a loop without a single failure or
timeout.

nixos/tests/keymap.nix

@@ -64,7 +64,6 @@ let
 
               # wait for reader to be ready
               machine.wait_for_file("${readyFile}")
-              machine.sleep(1)
 
               # send all keys
               for key in inputs: