…rue` (#4713)

## What are you changing in this pull request and why?

While responding to dbt-labs/dbt-core#9336, I
set `DO_NOT_TRACK=0` and examined the result. It does not behave the
same way as `DBT_SEND_ANONYMOUS_USAGE_STATS=True`.

Looking at the source code
[here](https://github.com/dbt-labs/dbt-core/blob/11cc71b75fa64b09888461339eb1eb3b394f9528/core/dbt/cli/flags.py#L252-L254)
explains why.

So we can safely just remove this line from the docs for
[`send_anonymous_usage_stats`](https://docs.getdbt.com/reference/global-configs/usage-stats)
to avoid confusion.

## Additional info

We first added support for the [Console Do Not
Track](https://consoledonottrack.com/) initiative within
dbt-labs/dbt-core#5000 as described in
dbt-labs/dbt-core#3540.

Any of the following are equivalent to
`DBT_SEND_ANONYMOUS_USAGE_STATS=False` (whether they are uppercase,
lowercase, or mixed case):
```
export DO_NOT_TRACK=1
export DO_NOT_TRACK=t
export DO_NOT_TRACK=true
export DO_NOT_TRACK=y
export DO_NOT_TRACK=yes
```

Any other values of `DO_NOT_TRACK` are ignored altogether and not have
any effect.

## Checklist
- [x] Review the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
so my content adheres to these guidelines.