remove collectd/df monitor

[atoulme]

Description:

This PR removes the collectd/df monitor. The monitor is deprecated and the filesystems monitor should be used instead. This component was deprecated 3 years 10 months ago.

Please see this deprecation notice on the filesystems monitor:

    ## Migrating from collectd/df
    The `collectd/df` monitor is being deprecated in favor of the `filesystems`
    monitor.  While the `collectd/df` monitor will still be available in
    5.0, it is recommended that you switch to the `filesystems` monitor soon
    after upgrading.  There are a few incompatibilities to be aware of between
    the two monitors:
     - `collectd/df` used a dimension called `plugin_instance` to identify the
       mount point or device of the filesystem.  This dimension is completely
       removed in the `filesystems` monitor and replaced by the `mountpoint`
       and `device` dimensions.  You no longer have to select between the two
       (the `reportByDevice` option on `collectd/df`) as both are always
       reported.
     - The mountpoints in the `plugin_instance` dimension of `collectd/df`
       were reported with `-` instead of the more conventional `/` separated
       path segments.  The `filesystems` monitor always reports mountpoints in
       the `mountpoint` dimension and uses the conventional `/` separator.
     - The `collectd/df` plugin set a dimension `plugin: df` on all datapoints,
       but `filesystems` has no such comparable dimension.

[rmfitzpatrick]

please replace w/ logged deprecation notice for at least a release

notice added