Remove class_exists checks for WP_Date_Query

[DJPaul]

We can remove the class_exists checks for the WP_Date_Query class, as that was introduced in WP 3.7 and our minimum is now 3.8.

[r-a-y]

In 10210:

Remove class_exists() check for the WP_Date_Query class.

BuddyPress 2.4.0 bumps our minimum WP version to WP 3.8, which supports
the WP_Date_Query class.

Props DJPaul.

Fixes #6621.