Remove the preload_member_roles feature flag

This feature flag was introduced in 17.9, which is more than 12 milestones ago.

• The feature flag has been disabled by default, and has been disabled on .com for a while now
• When the FF was enabled we noticed an uptick in the number of SQL queries because it was querying the custom role tables even when the user hadn't been assigned any custom roles
• Instead we preload custom roles in specific resolvers

TODO for the reviewers before merging this MR

• See the status of the rollout by checking #512871, https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/issues/?search=preload_member_roles&sort=created_date&state=all&label_name%5B%5D=host%3A%3Agitlab.com
• Verify the feature flag status via chatops by running /chatops gitlab run feature get preload_member_roles.
• Search for references to PreloadMemberRoles in frontend part of code
• Search for references to preload_member_roles in code
• Check if we need to remove any Gem or other related code by looking at the changes in !177717 (merged)

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