Remove unused Step Indicator component styles

[aduth]

🛠 Summary of changes

Removes unused step-indicator__step-subtitle styles.

Why?

• Unused code creates technical debt
• Reduce size of stylesheets
• Limit divergence with underlying U.S. Web Design System components
• Eliminate an instance of italics text
  • For context, this was discovered while auditing current use of italic text, since italic text is not intended to be supported for Chinese content.).

Added in #6817, last reference removed in #7623.

📜 Testing Plan

Verify no regressions in the appearance of Step Indicator in the identity verification flow.