Remove SYSTEM_COMPOSER

[codygunton]

This SYSTEM_COMPOSER variable was introduced by @suyash67 to ease the process of changing the underlying proof system of the Aztec Connect rollup (pre-root verifier circuit). It seems to me this feature is now just noise, so I propose to remove it from Barretenberg. It's possible that we should reinstate it in some other form in Aztec 3, though there we will be blending various proof systems and curves, so something more robust will be needed. Thoughts @suyash67 @iAmMichaelConnor?