Unnecessary $sites count condition in multisite load process

[jeremyfelt]

​mu:1422 moved a check for single network vs multi-network into wpmu_current_site(). Existing code inside wpmu_current_site() intended for a $sites count of 1 become inaccessible.

In [12921], the remaining count checks were consolidated into one, but the instance of 1 == count( $sites ) in that scenario is still not accessible.

This doesn't hurt anything and the likelihood of this part of the function being reached is so extremely low. It does cause a double take. And made me investigate it.

[jeremyfelt]

Closing in favor of the work being done in #27003