Administration: Add `doing_wp_cron` to `wp_removable_query_args()`

[dlh]

In my experience, the doing_wp_cron query var that appears when ALTERNATE_WP_CRON is enabled can cause user confusion in the admin. As far as I can tell, it isn't used by the browser or needed as part of the canonical URL.

While doing_wp_cron can also appear on the frontend, where it's immune to wp_admin_canonical_url(), the attached patch would at least suppress it where core is able to do so reliably.

[SergeyBiryukov]

In 47002:

Administration: Add doing_wp_cron to wp_removable_query_args().

It's a single-use URL parameter that does not need to be passed to canonical URLs in the admin.

Props dlh.
Fixes #49017.