Update README to remove .env file instructions

[islero]

Removed instructions for creating a .env file in the README.

Description

Brief description of the changes in this PR.

Type of Change

• 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)
• Documentation update
• Performance improvement
• Code refactoring

Related Issues

Fixes #(issue number)
Related to #(issue number)

Changes Made

• Change 1
• Change 2
• Change 3

Testing

Describe the tests you ran to verify your changes:

• Test A
• Test B

Test Configuration

• Python version:
• OS:
• NautilusTrader version:

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Screenshots (if applicable)

Add screenshots to help explain your changes.

Additional Notes

Any additional information that reviewers should know.