remove the -Z no-opt flag

[thestinger]

This seems very pointless because you can just not pass optimization flags. It might date from a time when optimizations had frontend implications.

[huonw]

I use -Z no-opt to temporarily avoid the expense of -O for specific targets makefile projects (including/especially mozilla/rust) i.e. make foo RUSTFLAGS='-Z no-opt'.

This can be equally well simulated if #7493 was fixed to allow multiple --opt-levels (since RUSTFLAGS='--opt-level=0' works equally well).

(This is just a minor thing, but it is often easier to use the makefile targets than to work out the correct manual rustc invocation.)