[TASK] Remove config.index_enable usages

[dkd-kaehm]

What should be done in the scope of this task?

The setting config.index_enable is for pages only. EXT:solr does not set to 1 as default. That makes it hard to find the cause of errors for page indexing, even it is in the docs and in FAQ.

The setting must be removed or apply for all index-queues.

I would propose to remove this setting, because Site-Configs "Enable Solr for this site"/(solr_enabled_read) does it already.

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Target version 13.0.0