Documentation Update: Removal of linux exclusive fields from system integration

[ritalwar]

Update the documentation for the system integration to remove references to the system.diskio.iostat and few system.memory fields since they have been replaced and should now be fetched using the Linux integration. While the changes in beats were implemented quite some time ago, the integration documentation has not been updated, which lead to user confusion and the creation of SDHs like this.

The removal of these fields in beats was carried out through this PR, which can serve as a reference for updating the fields in the system integration.