Remove graphql_resolver_complexity flag

What does this MR do?

based on logs from last 20 hours, overall complexity was stable on gitlab.com - although average complexity increased (as expected), there are no queries where the complexity limit would reach our 200 limit (though boards queries are pretty close, but I don't think this is a blocker for removing the flag - more details/link to kibana in #35736 (comment 307867623)).

Closes #208120 (closed) Closes #35736 (closed)

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