fix: LEAP-381: Remove excess regexp from BEM module

[hlomzik]

It's not used in LSF anyway. And the fix is pretty simple and straightforward.

PR fulfills these requirements

• [ ] Tests for the changes have been added/updated
• [ ] Docs have been added/updated
• [ ] Best efforts were made to ensure docs/code are concise and coherent (checked for spelling/grammatical errors, commented out code, debug logs etc.)
• [x] Self-reviewed and ran all changes on a local instance

Describe the reason for change

To fix regexp injection vulnerability https://github.com/HumanSignal/label-studio-frontend/security/code-scanning/9

This change affects (describe how if yes)

• [ ] Performance
• [x] Security
• [ ] UX

Does this PR introduce a breaking change?

• [ ] Yes, and covered entirely by feature flag(s)
• [ ] Yes, and covered partially by feature flag(s)
• [x] No
• [ ] Not sure (briefly explain the situation below)

What level of testing was included in the change?

• [ ] e2e (codecept)
• [ ] integration (cypress)
• [ ] unit (jest)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (88ba15c) 68.27% compared to head (fcd0cae) 68.28%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1648 +/- ## ======================================= Coverage 68.27% 68.28% ======================================= Files 443 443 Lines 28690 28690 Branches 7633 7630 -3 ======================================= + Hits 19589 19591 +2 + Misses 7849 7847 -2 Partials 1252 1252 ```

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[hlomzik]

/git merge master

Workflow run Successfully pushed new changes: Merge remote-tracking branch 'origin/master' into fb-leap-381/css-prefix (e9010cfb40f4df376de258c4bf87da9abe884596)