Remove unused Transportation icons

[coliff]

Description

The Envelope, Export, Triangle-Left and Triangle-Right icons in the Transportation set are not used because they exist in Modus-Solid which is used when the icons get built.

Removing them saves a bit of time and keeps the build process a little bit quieter.

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

How Has This Been Tested?

Building locally generates the exact same output as these icons were not used at all anyway. No changes are expected as part of this PR.

Checklist

• My code follows the style guidelines of this project
• 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 in downstream modules
• I have checked my code and corrected any misspellings

[github-actions]

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