ReadingDepth: Remove ReadingDepth instrument

... and associated config variables and documentation.

Per T229042, the instrument was disabled since 20th August 2019.
However, the instrument and associated (but null) config variables have
been sent to the all RL-capable clients to be parsed ever since.

Bug: T229042
Change-Id: I490981502b259099f6faf1ec68a605b9aa5ebb6b