refactor: Drop run_shell_command()

[zulinx86]

Follow up of PR #4689.

Changes

run_shell_command() is no longer used and related error variant and tests can be dropped.

Reason

$ tools/devtool build --release
[Firecracker release.sh 2024-07-18T15:27:38+00:00] Building version=v1.9.0-dev, profile=release, target=x86_64-unknown-linux-musl, Rust toolchain=1.76.0...
   Compiling jailer v1.9.0-dev (/firecracker/src/jailer)
   Compiling vmm v0.1.0 (/firecracker/src/vmm)
   Compiling firecracker v1.9.0-dev (/firecracker/src/firecracker)
   Compiling cpu-template-helper v1.9.0-dev (/firecracker/src/cpu-template-helper)
   Compiling snapshot-editor v1.9.0-dev (/firecracker/src/snapshot-editor)
warning: variant `ShellCommand` is never constructed
  --> src/cpu-template-helper/src/fingerprint/dump.rs:20:5
   |
12 | pub enum FingerprintDumpError {
   |          -------------------- variant in this enum
...
20 |     ShellCommand(String, String),
   |     ^^^^^^^^^^^^
   |
   = note: `FingerprintDumpError` has a derived impl for the trait `Debug`, but this is intentionally ignored during dead code analysis
   = note: `#[warn(dead_code)]` on by default

warning: function `run_shell_command` is never used
  --> src/cpu-template-helper/src/fingerprint/dump.rs:64:4
   |
64 | fn run_shell_command(cmd: &str) -> Result<String, FingerprintDumpError> {
   |    ^^^^^^^^^^^^^^^^^

warning: `cpu-template-helper` (bin "cpu-template-helper") generated 2 warnings
    Finished release [optimized] target(s) in 47.46s

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license. For more information on following Developer
Certificate of Origin and signing off your commits, please check
CONTRIBUTING.md.

PR Checklist

• [ ] If a specific issue led to this PR, this PR closes the issue.
• The description of changes is clear and encompassing.
• [ ] Any required documentation changes (code and docs) are included in this
  PR.
• [ ] API changes follow the Runbook for Firecracker API changes.
• [ ] User-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.
• [ ] New TODOs link to an issue.
• Commits meet
  contribution quality standards.

---

• This functionality cannot be added in rust-vmm.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.08%. Comparing base (643349a) to head (d30a1fe).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4690      +/-   ##
==========================================
- Coverage   82.09%   82.08%   -0.02%     
==========================================
  Files         255      255              
  Lines       31330    31312      -18     
==========================================
- Hits        25719    25701      -18     
  Misses       5611     5611              

Flag	Coverage Δ
5.10-c5n.metal	82.15% <ø> (-0.02%)	⬇️
5.10-m5n.metal	82.14% <ø> (-0.01%)	⬇️
5.10-m6a.metal	81.44% <ø> (-0.02%)	⬇️
5.10-m6g.metal	79.43% <ø> (-0.02%)	⬇️
5.10-m6i.metal	82.13% <ø> (-0.02%)	⬇️
5.10-m7g.metal	79.43% <ø> (-0.02%)	⬇️
6.1-c5n.metal	82.15% <ø> (-0.02%)	⬇️
6.1-m5n.metal	82.13% <ø> (-0.02%)	⬇️
6.1-m6a.metal	81.44% <ø> (-0.02%)	⬇️
6.1-m6g.metal	79.43% <ø> (-0.02%)	⬇️
6.1-m6i.metal	82.13% <ø> (-0.02%)	⬇️
6.1-m7g.metal	79.43% <ø> (-0.02%)	⬇️

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