tests: Fix DTS overlay for build_all/input

It's clearly a bug: an input device is not meant to listen to itself.
This creates a dependency loop, and it is actually surprising this did
not get detected by DTS (which does call add_edge on graph on the same
node).

Binding description of the node says 'input' property can be left empty
so removing the line entirely.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@proton.me>

tests/drivers/build_all/input/app.overlay

@@ -145,7 +145,6 @@
 		};
 
 		longpress: longpress {
-			input = <&longpress>;
 			compatible = "zephyr,input-longpress";
 			input-codes = <0>;
 			short-codes = <0>;