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

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

Change-Id: I8f55517b0e4712cb1ae07cafc69aca9cec1d6306