Revert "Capture delayed events and cancel as necessary."

Tag #1052 (backout).
Backed out for creating issues like #1191 and other issue-less problems
with e.g. losing the caret from text boxes. Of note: this was also
backed out from m-c for similar reasons in Bug 1332433.
This fixes #1191.
This reverts commit 3424afccaebc71c4acd4fa9eadf519c408c5965b.

toolkit/components/passwordmgr/test/subtst_master_pass.html

@@ -2,6 +2,11 @@
 This form triggers a MP and gets filled in.<br>
 <form>
 Username: <input type="text"     id="userfield" name="u"><br>
-Password: <input type="password" id="passfield" name="p"
-                 oninput="parent.postMessage('filled', '*');"><br>
+Password: <input type="password" id="passfield" name="p"><br>
+<script>
+    // Only notify when we fill in the password field.
+    document.getElementById("passfield").addEventListener("input", function() {
+        parent.postMessage("filled", "*");
+    });
+</script>
 </form>