cmd/gb: remove -goroot flag
cmd/gb: remove -goroot flag

Update #28

The toolchain goroot always defaults to runtime.GOROOT.

tbruyelle/gb,constabulary/gb,inconshreveable/gb,hyper-carrot/gb,mopemope/gb,acasajus/gb,URXtech/gb,constabulary/gb,lparkes/gb,hyper-carrot/gb,constabulary/gb,tbruyelle/gb,davidkaya/gb,guiquanz/gb,ches/gb,davidkaya/gb,ches/gb,mopemope/gb,mopemope/gb,inconshreveable/gb,MerlinDMC/gb,tianon/gb,Luit/gb,tianon/gb