[CI] Remove “Install ShellCheck” step from ShellCheck GitHub Actions workflow

Summary:
Because ShellCheck comes pre-installed on the GitHub Actions virtual environment we're using for this workflow (ubuntu-latest), we don't need to include the installation commands for ShellCheck.

Read more on the GitHub Actions Virtual Environments README for ubuntu-latest, under the “Installed apt packages” section.

Test Plan: Land this diff and see if the workflow executes as expected on GitHub.

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, tomek, adrian

Differential Revision: https://phab.comm.dev/D4756