pw_presubmit: Remove shellcheck from lintformat program

This is failing on Pigweed, but most of us don't notice because we don't
have shellcheck installed so it isn't run. Also, shellcheck is too
defensive—we don't need to handle arbitrary variable values when we set
the variable ourselves a few lines above.

Leaving it in so downstream projects can use it if they want, and people
working on Pigweed can still manually run it.

Change-Id: I02e54cc74d9de8ed777fc580a3d0ab659281ebcf
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/213617
Pigweed-Auto-Submit: Rob Mohr <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>
Presubmit-Verified: CQ Bot Account <[email protected]>
Lint: Lint 🤖 <[email protected]>
Reviewed-by: Armando Montanez <[email protected]>

pw_presubmit/py/pw_presubmit/pigweed_presubmit.py

@@ -1413,6 +1413,7 @@ def owners_lint_checks(ctx: PresubmitContext):
     pw_transfer_integration_test,
     python_checks.update_upstream_python_constraints,
     python_checks.vendor_python_wheels,
+    shell_checks.shellcheck,
     # TODO(hepler): Many files are missing from the CMake build. Add this check
     # to lintformat when the missing files are fixed.
     source_in_build.cmake(SOURCE_FILES_FILTER, _run_cmake),
@@ -1476,7 +1477,6 @@ def owners_lint_checks(ctx: PresubmitContext):
         SOURCE_FILES_FILTER_GN_EXCLUDE
     ),
     source_is_in_cmake_build_warn_only,
-    shell_checks.shellcheck if shutil.which('shellcheck') else (),
     javascript_checks.eslint if shutil.which('npm') else (),
     json_check.presubmit_check,
     keep_sorted.presubmit_check,