[RFCLAT#6] clatd_test: remove dead test for detect_mtu am: ed36b4b664 am: 900d2d81b9 am: ce3432abc0 am: 5f8e48b6bf

Original change: https://android-review.googlesource.com/c/platform/external/android-clat/+/1903811

Change-Id: I16e32b8ad612841f4d5b6bfe9dc8b8e39b0f3f05