vulncheck: remove additional meaningless string sorting for stacks
vulncheck: remove additional meaningless string sorting for stacks

String ordering on stacks was employed for deterministic output which
cannot happen anyhow since the initial slice of stacks is not
determinstic. The string comparison is expensive, especially for large
vulnerability graphs.

This shaves off approx. 15-20 seconds for k8s and vault projects.

Change-Id: I3c4502b948eb32ebf2c4705097c70e3928bbf3de
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/410897
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Zvonimir Pavlinovic <490af24f3def1258d2f7e9b10b20e341f6c68320@google.com>
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/vuln,golang/vuln