Remove the disabled styling from Input

This seems to be a hangover from the early days of the development of
`Input`, and the styles do nothing as there's nothing else in the `Input`
code that makes use of the class that's involved.

Removed in anticipation of #1748 taking care of this.

src/textual/widgets/_input.py

@@ -110,9 +110,6 @@ class Input(Widget, can_focus=True):
         height: 1;
         min-height: 1;
     }
-    Input.-disabled {
-        opacity: 0.6;
-    }
     Input:focus {
         border: tall $accent;
     }
@@ -121,9 +118,6 @@ class Input(Widget, can_focus=True):
         color: $text;
         text-style: reverse;
     }
-    Input>.input--placeholder {
-        color: $text-disabled;
-    }
     """
 
     cursor_blink = reactive(True)