[alerts] — Remove inner span

[ffoodd]

The duplicated cross when upgrading to Boosted 4.4.1 is due to a diff between Boosted and Bootstrap (thus ng-bootstrap).

The current Boosted pattern is:

<div class="alert alert-lg alert-danger alert-dismissible fade show mb-0" role="alert">
    <span class="alert-icon"><span class="sr-only">Danger</span></span>
    <div>
        <h4 class="alert-heading">Error notification text goes here.</h4>
        <p>Description text goes here.</p>
    </div>
    <button type="button" class="close" data-dismiss="alert">
        <span class="sr-only">Close alert message</span>
    </button>
</div>

There's a first step to remove the unwanted cross, in CSS:

.close > [aria-hidden="true"]  {
    display: none;
}

Which will solve the visual inconsistency.

However there's a second step which should be done to match Boosted, but it requires to override the generated markup from ng-bootstrap's alert.