Remove vendor.conf as it is no longer used

[thaJeztah]

@cquon @justincormack @HuKeping PTAL

[HuKeping]

/go/src/github.com/theupdateframework/notary/cmd/notary/keys.go,525,"Deferring unsafe method ""*os.File"" on type ""Close""",MEDIUM,HIGH,defer from.Close(),CWE-
/go/src/github.com/theupdateframework/notary/cmd/notary/keys.go,554,"Deferring unsafe method ""*os.File"" on type ""Close""",MEDIUM,HIGH,defer f.Close(),CWE-
/go/src/github.com/theupdateframework/notary/cmd/notary/util.go,50,Expect WriteFile permissions to be 0600 or less,MEDIUM,HIGH,"ioutil.WriteFile(t.output, payload, 064

Shall we use a defer function to close the os.File?

    from, err := os.Open(file)
    if err != nil {
        return err
    }
    defer from.Close()              // this seems not a good practice and gosec shouting.

wondering why this PR trigger this warning

[thaJeztah]

Oh! It's the same as #1537 (comment). The version of the gosec linter is not pinned, and they recently did a new release.

I wanted to keep this change separate as it seemed more clean, but perhaps I should do a quick carry of that first commit

[thaJeztah]

Opened #1540 with a carry. Will rebase this one after that

[HuKeping]

Thanks @thaJeztah !

LGTM!