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js: Remove keypress handler

At least in modern browsers (and I suspect in all browsers), cancelling
a keydown event ensures that the subsequent keypress event doesn't fire.
See <https://w3c.github.io/uievents/#keys-cancelable-keys>.

So there's no point in having a handler on keypress events that just
tries to cancel them as well.  Removing the handler doesn't do much now,
but it opens the possibility of being a bit more selective about which
keydown events we cancel.

--- a/emccpre.js

+++ b/emccpre.js

@@ -304,9 +304,8 @@

         }

     };

-    // Set up keyboard handlers. We do all the actual keyboard

-    // handling in onkeydown; but we also call event.preventDefault()

-    // in both the keydown and keypress handlers. This means that

+    // Set up keyboard handlers. We call event.preventDefault()

+    // in the keydown handler. This means that

     // while the canvas itself has focus, _all_ keypresses go only to

     // the puzzle - so users of this puzzle collection in other media

     // can indulge their instinct to press ^R for redo, for example,

@@ -316,9 +315,6 @@

     onscreen_canvas.onkeydown = function(event) {

         key(event.keyCode, event.key, event.char, event.location,

             event.shiftKey ? 1 : 0, event.ctrlKey ? 1 : 0);

-        event.preventDefault();

-    };

-    onscreen_canvas.onkeypress = function(event) {

         event.preventDefault();

     };