Remove active timestamp from header when on a BuddyPress profile page.

The active timestamp in the header-top.php template uses
<div class="activity"> as its HTML wrapper.

When the active timestamp is outputted and when you're on a user profile's
Activity page, if a user selects a dropdown filter, the layout of the page
will be broken because the AJAX response tries to inject the contents into
the active timestamp's <div class="activity"> block instead of in the other
<div class="activity"> block that renders later on the page.

Since the last active timestamp isn't visible to begin with (it's hidden
with CSS at the moment), this commit outright removes the active timestamp
from the header.