Duplicate 'ajaxurl' variables

[r-a-y]

As I was looking into #4669, in bp-legacy, the 'ajaxurl' is outputted in 'bp_head':
​https://buddypress.trac.wordpress.org/browser/trunk/bp-templates/bp-legacy/buddypress-functions.php#L95

The problem with that is the 'ajaxurl' variable is already defined in bp_core_add_ajax_url_js().

We could either:

1. remove BP_Legacy::head_scripts()
2. remove bp_core_add_ajax_url_js() and add a similar 'ajaxurl' function in bp-default's functions.php
3. rename the head scripts hook in bp-legacy to use 'wp_head' instead of 'bp_head' - would result in duplicate 'ajaxurl' variables being outputted when a theme other than bp-default is enabled

If we are going to stick with having the head_scripts() method in the BP_Legacy class, I would rather see it in the parent BP_Theme_Compat class instead so other theme packs like Turtleshell will not have to define this in their own class unless they want to override it.

There will always be duplicate ajaxurl's the wp_head, as long as there are plugins like BuddyPress, bbPress, Jetpack, etc... installed. Since these are in the global JS scope, and they all point to the same place, I don't see this as a huge deal.

Long term, we should have our own theme-side AJAX handler. See #4449.