Firedrake UFL has diverged from FEnICS UFL

[ReubenHill]

#23 was accidentally merged to Firedrake UFL but has not been merged to FEnICS UFL (changes have been requested - see FEniCS#107). We ought to remove that/those commits from the Firedrake UFL fork.

[ReubenHill]

I believe that, because of this, we missed firedrakeproject/firedrake#2595 becoming an issue