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

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

Change-Id: I63fb12645d0341cc55cd33c3eb45999383cf838b