Can throttle control be considered as a primary flight control?
14 CFR 23.673 - Primary flight controls defines primary flight controls as:


  Primary flight controls are those used by the pilot for the immediate control of pitch, roll, and yaw.


While a throttle could be used to effect changes in aircraft attitude (depending on configuration and other factors), the effect could hardly be called immediate/direct. So, throttle could hardly be called a primary flight control; and as @Ron pointed out, there are gliders.

Note: I'm aware that aircraft have been controlled solely using throttle. But it is an exception rather than the norm.