nimble/ll/dtm: Remove scheduler first when stopping DTM
nimble/ll/dtm: Remove scheduler first when stopping DTM

It should be safer to begin stopping DTM by removing scheduler item so
it does not fire while we are stopping things.

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble