[Uptime] Saved Objects interaction is performed with an unnecessary custom client

[justinkambic]

Kibana version:
master

Elasticsearch version:
master

Server OS version:
macOS

Browser version:
Chrome 77

Browser OS version:
macOS

Original install method (e.g. download page, yum, from source, etc.):
from source

Describe the bug:
When we implemented a Saved Objects client in the Uptime app we created our own custom client, rather than utilizing the provided getter method available on http requests. This can lead to things like ignoring Spaces when making requests, among other pitfalls.

Steps to reproduce:
This is more of an implementation mistake than a bug that breaks functionality. There should be no noticeable difference in the user's interaction with Kibana after the issue is fixed.

Expected behavior:
The Uptime app's server code should simply use the getter function supplied as part of the request object.

Screenshots (if relevant):
N/A

Errors in browser console (if relevant):
N/A

Provide logs and/or server output (if relevant):
N/A

Any additional context:
N/A

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)

[justinkambic]

This was resolved by #51403.