Remove instruction to install a Javascript runtime

[orien]

Change

Since #506, the test suite no longer requires a Javascript runtime. Remove the instruction to install one from the README document.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue).
• [ ] New feature (non-breaking change which adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• [ ] I've added tests for my code.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.