chore: remove mock from top level simapp app.go

[colin-axner]

Description

I was going through old issues and thought I'd take care of this one real quick

closes: #7066

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against the correct branch (see CONTRIBUTING.md).
• Linked to GitHub issue with discussion and accepted design, OR link to spec that describes this work.
• Code follows the module structure standards and Go style guide.
• Wrote unit and integration tests.
• Updated relevant documentation (docs/).
• Added relevant godoc comments.
• Provide a conventional commit message to follow the repository standards.
• Include a descriptive changelog entry when appropriate. This may be left to the discretion of the PR reviewers. (e.g. chores should be omitted from changelog)
• Re-reviewed Files changed in the GitHub PR explorer.
• Review SonarCloud Report in the comment section below once CI passes.

[sonarcloud]

Quality Gate passed for 'ibc-go'

Issues
149 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud