MNT: Remove internal usage of the 'backend' key in rcParams - #32495

PR summary

Working towards #31791.

This removes most internal usages of the 'backend' key (some usage in tests is still in place because I have to decide there whether we still want to test the key API).

It's conceptually a schematic replacement

• rcParams._get('backend') -> rcParams._backend - obtain raw backend value
• x = rcParams['backend'] -> x = rcParams._get_backend_resolved() - obtain resoved backend
• rcParams['backend'] = x -> rcParams._backend = x - store the value. Note that this write had no specific logic attatched. It's only be used to store the new backend value.

Note: There have been some places writing rcParamsDefault['backend'] and rcParamsOrig['backend']. These were needed to synchronize and persist backend information even after resetting rcParams. That's not needed anymore and thus removed now that the backend information only exists once as class attribute on RcParams.

Note2: In theory it would even be better to use RcParams._backend rather than access via instance rcParams._backend backend. I've not done that for now, because it would complicate the PR. We may do this in a followup.

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