[nrfconnect] Fix memory leak in Window Covering sample.

We were leaking 8 bytes each time the new attribute change was
reported by the WindowCovering Server. It was resulting in
running out of heap when stressing the device (by sending multiple
up-or-open/down-or-close commands).

Signed-off-by: Marcin Kajor <[email protected]>

examples/window-app/nrfconnect/main/WindowCovering.cpp

@@ -343,4 +343,6 @@ void WindowCovering::DoPostAttributeChange(intptr_t aArg)
     VerifyOrReturn(data != nullptr);
 
     PostAttributeChange(data->mEndpoint, data->mAttributeId);
+
+    chip::Platform::Delete(data);
 }