Remove asset tag helper patch assigning empty image alt

[aduth]

🛠 Summary of changes

Removes a patched image_alt method defined to override Rails' default asset tag helpers, originally added in #3350.

Why?

• This method no longer appears to exist or be used in Rails (Do not generate default alt text for images rails/rails#30213)
  • Deprecated in 5.2, removed in 6.0
• As described in the pull request deprecating the method, auto-generating text alternatives is an anti-pattern, and we should always explicitly assign one. Defaulting to an empty alt attribute is a bad accessibility practice.
  • Fortunately, for pages where expect_page_to_have_no_accessibility_violations accessibility checks are run, we should already account for absence of any alt attributes, via Axe's "Images must have alternative text" rule

📜 Testing Plan

Verify build passes.

[jmdembe]

Removing the helper makes sense.