metric-backend: hide the pill remove button while editing

The group-by pill and its "then" step pills showed a remove (X) button
even while open for editing, where a stray click could discard the pill
the user was in the middle of adjusting.

Show the remove button only while the pill is collapsed, so clearing a
grouping or dropping a step stays a deliberate action.

Change-Id: If01a29c47b6182d7fc8ef874f8aac52cd8724f5d