fix(input): allow pointer events on placeholder (#3878)

Removes the `pointer-events: none` from the input placeholder in order to allow for users to bind events to it. Clicking through isn't necessary anyway, because it automatically redirects focus to the underlying input already.

src/lib/input/input-container.scss

@@ -132,7 +132,6 @@ $mat-input-underline-disabled-background-image:
   top: 0;
 
   font-size: 100%;
-  pointer-events: none;  // We shouldn't catch mouse events (let them through).
   z-index: 1;
   padding-top: 1em;
 
@@ -181,7 +180,6 @@ $mat-input-underline-disabled-background-image:
   width: 100%;
   padding-top: 1em;
   overflow: hidden;
-  pointer-events: none;  // We shouldn't catch mouse events (let them through).
   transform: translate3d(0, 0, 0); // Prevents the label from shifting after the animation is done.
 
   // Keeps the element height since the placeholder text is `position: absolute`.