Delete two Sidekiq reliable_fetcher flags

What does this MR do and why?

gitlab_sidekiq_enable_semi_reliable_fetcher

• The gitlab_sidekiq_enable_semi_reliable_fetcher flag was introduced by gitlab-foss!23854 (diffs) on 2018-12-17

• No associated FF state change issues at https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/issues/?search=gitlab_sidekiq_enable_semi_reliable_fetcher&sort=created_date&state=all&label_name%5B%5D=host%3A%3Agitlab.com

• I confirmed it doesn't exist on GitLab.com:

  The feature gitlab_sidekiq_enable_semi_reliable_fetcher does not exist on gprd, but is default_enabled: true in their YAML definition.

gitlab_sidekiq_reliable_fetcher

• The gitlab_sidekiq_reliable_fetcher was introduced by 34687cf0 on 2018-09-12

• No associated FF state change issues at https://gitlab.com/gitlab-com/gl-infra/feature-flag-log/-/issues/?search=gitlab_sidekiq_reliable_fetcher&sort=created_date&state=all&label_name%5B%5D=host%3A%3Agitlab.com

• I confirmed it's enabled globally on GitLab.com

  

MR acceptance checklist

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

config/feature_flags/ops/gitlab_sidekiq_enable_semi_reliable_fetcher.yml

----
-name: gitlab_sidekiq_enable_semi_reliable_fetcher
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab-foss/-/merge_requests/23854
-rollout_issue_url: 
-milestone: '11.6'
-type: ops
-group: group::geo
-default_enabled: true