fix(NODE-7270): remove extra BSONType file in docs/Next/variables

The fact that we have two files in this directory which only differ
by casing messes up checkouts on case-insensitive filesystems, like
the default macOS and Windows filesystems.

We don't need the `BSON.BSONType-1.html` file; `BSON.bsonType-1.html`
and `BSON.BSONType-2.html` are the correct files, and
`BSON.BSONType-1.html` is merely an outdated version of the latter
that stuck around from before the addition of lowercase `bsonType`
to the API.

According to our git history, no file from `docs/Next` has ever
been deleted; as a follow-up item, we should probably make sure
that when we generate docs, we start out with a fresh, empty
directory, instead of only adding new files.

Author: addaleax