LU-15235 tests: skip sanity/56od in interop

LU-15235 tests: skip sanity/56od in interop

Sanity test_56oc and test_56od were using the btime_supported()
function to check it "lfs find" supported file birth time, but
this did not properly check whether the MDS supported this option.

Remove the btime_supported() check and just use the version, since
this has been around a few releases already.

Fixes: 186b97e68abb ("LU-11971 utils: Send file creation time to clients")
Test-Parameters: trivial testlist=sanity serverversion=2.12.9 env=ONLY=56
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I0c85103c843d3b993e3e112bf5d0da976d3ebbe5