Remove external/libiio/.gitmodules and submodule directories. am: 953ed47bb9 am: f04dc70911 am: 3b6dd9ebaa am: 5c9cddef4c

Original change: https://android-review.googlesource.com/c/platform/external/libiio/+/1830468

Change-Id: Ic9ab1681601907175cbe4a5e9e0aa50f1b7151eb