Remove the undocumented string form of link-external-icon #15261

Description

The external-link script in quarto-html-after-body.ejs still has a branch for a string value of link-external-icon.
With a string, it inserts a <i class="bi-<value> quarto-ext-icon"> after each external link instead of using the CSS icon.

This form comes from the original implementation (fc1c503) and was never documented.
The schema accepts only true or false, so a string reaches this branch only with validate-yaml: false.
In #9469 the branch got a sibling check, so it no longer adds a second icon when the script runs again after pagination, search or sort.

The proposal is to remove the string branch and keep the boolean behaviour as it is.

• Related: fix: link-external-newwindow: true not respected by default listing #9469