Remove unused epic related boards code [RUN AS-IF-FOSS]

What does this MR do?

Because we are using the newly created dropdown widget for epics in boards, we can remove the code previously used by board_sidebar_epic_select.vue.

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?) has no user facing changes.
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• 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.