[DevOps Adoption] Remove obsolete Segment selections table

What does this MR do?

removes unused table. It's obsolete since %13.9 (#299889 (closed)) so can be safely removed.

Migration output

== 20210526160133 RemoveSegmentSelectionsTable: reverting =====================
-- create_table(:analytics_devops_adoption_segment_selections)
   -> 0.0591s
== 20210526160133 RemoveSegmentSelectionsTable: reverted (0.0593s) ============

== 20210526160133 RemoveSegmentSelectionsTable: migrating =====================
-- drop_table(:analytics_devops_adoption_segment_selections)
   -> 0.0128s
== 20210526160133 RemoveSegmentSelectionsTable: migrated (0.0130s) ============

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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Related to #321534 (closed)