Remove rusti reference from pp.mk
auto merge of #9992 : astrieanna/rust/rm_rusti, r=alexcrichton

It seems like rusti has been removed, except for one reference in one Makefile. This reference breaks building rust on my computer because the "all-target" rule has rusti as a target.

~~~~
make: *** No rule to make target `x86_64-unknown-linux-gnu/stage2/bin/rusti', needed by `all-target-x86_64-unknown-linux-gnu-host-x86_64-unknown-linux-gnu'.  Stop.
~~~~

Removing this line fixes things for me.