cmd/vet: drop column information from error
cmd/vet: drop column information from error

The column information can be misleading.

R=golang-dev, dsymonds, r
CC=golang-dev
https://codereview.appspot.com/7300100

pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,dsriram/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,dsriram/golang,tv42/old-go,gnanderson/go,apparentlymart/golang,tv42/old-go,dsriram/golang,dsriram/golang