stats: revert docs for new --stat-tag CLI arg (#364)

Revert "stats: docs for new --stat-tag cli arg (#361)"

This reverts commit 06ae0116c1.

Signed-off-by: Taiki Ono <taiks.4559@gmail.com>

docs/root/operations/cli.rst

@@ -132,14 +132,6 @@ following are the command line options that Envoy supports.
   on AWS, `Zone <https://cloud.google.com/compute/docs/regions-zones/>`_ on GCP,
   etc.
 
-.. option:: --stats-tag <string>
-
-  *(optional)* Defines :ref:`stats <arch_overview_statistics>` tags which will
-  be added to all metrics as tags. To add multiple tags, specify this option
-  multiple times. The tag string must have a semicolon separated format like
-  `name:value`. The tags will be only available with :ref:`stats sinks
-  <envoy_api_msg_StatsSink>` which support tagged metrics: e.g.
-  :ref:`envoy.dog_statsd <envoy_api_msg_DogStatsdSink>`.
 
 .. option:: --file-flush-interval-msec <integer>