fix: cleanup Gemfile

[CAMOBAP]

• https://github.com/metanorma/metanorma-standoc/issues/846

[codecov[bot]]

Codecov Report

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

Comparison is base (812a549) 95.19% compared to head (4362ef1) 95.19%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #84 +/- ## ======================================= Coverage 95.19% 95.19% ======================================= Files 7 7 Lines 541 541 ======================================= Hits 515 515 Misses 26 26 ```

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