metric-backend: unify group-by remove UX with then steps

The chained then-step pills expose a close (X) button to remove them,
but the group-by pill did not: clearing a grouping meant opening the
pill and picking "no grouping" from the dropdown.

Give the group-by pill the same close button so an active grouping can
be cleared in one click. It appears only while a grouping is active,
since there is otherwise nothing left to remove.

Change-Id: Idc074b31d3b81fc722792c467d634ecd1f9e9f49