Remove fs.s3a.executor.capacity

When s3guard was part of s3a, DynamoDBMetadataStore was the only consumer of StoreContext that used throttled executor provided by StoreContext, which internally uses fs.s3a.executor.capacity to determine executor capacity for SemaphoredDelegatingExecutor. With the removal of s3guard from s3a, we should also remove fs.s3a.executor.capacity and it's usages as it's no longer being used by any StoreContext consumers. The config's existence and its description can be really confusing for the users.