Remove mention of repositories.s3.base_path in doc (#69516)

We removed the global `repositories.s3.base_path` setting in 6.0 but it
is still mentioned in the docs for the S3 repository plugin. This commit
removes it from the docs.

Relates #24445

docs/plugins/repository-s3.asciidoc

@@ -196,10 +196,9 @@ The following settings are supported:
 
 `base_path`::
 
- Specifies the path within bucket to repository data. Defaults to value of
- `repositories.s3.base_path` or to root directory if not set. Previously,
- the base_path could take a leading `/` (forward slash). However, this has
- been deprecated and setting the base_path now should omit the leading `/`.
+ Specifies the path to the repository data within its bucket. Defaults to an
+ empty string, meaning that the repository is at the root of the bucket. The
+ value of this setting should not start or end with a `/`.
 
 `chunk_size`::