Remove the infinitely_collapsible_sections feature flag

What does this MR do?

This removes the infinitely_collapsible_sections feature flag

Rollout: #335297 (closed) Original issue: #29766

Screenshots or Screencasts (strongly suggested)

Screen_Recording_2021-07-06_at_10.17.34

How to setup and validate locally (strongly suggested)

1. Visit any job log
2. Click any collapsible section, it should collapse all the way as intended.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• 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.
• This change is backwards compatible across updates, or this does not apply.