fix/batch-changes: drop orphaned hook files parse test

The hook-files parse-time validation lives in lib/batches (synced from the
sourcegraph monorepo) and was intentionally removed from this PR in
996bcec. The service_test case for it remained, so ParseBatchSpec returned
no error and the test nil-dereferenced err.Error(). The executor sink check
in createFilesToMount (and its test) remains as src-cli's defense.

Author: cbrnrd

internal/batches/service/service_test.go

@@ -615,31 +615,6 @@ changesetTemplate:
 `,
 			expectedErr: errors.New("parsing batch spec: step 1 files target path contains invalid characters"),
 		},
-		{
-			name:         "hook files target path with comma",
-			batchSpecDir: tempDir,
-			rawSpec: `
-version: 3
-name: test-spec
-description: A test spec
-on:
-  - repository: github.com/sourcegraph/src-cli
-changesetTemplate:
-  title: Test Hook Files
-  body: Test hook files target path with comma
-  branch: test
-  commit:
-    message: Test
-changesetHooks:
-  onCIFailure:
-    steps:
-      - run: echo test
-        image: alpine:3
-        files:
-          "/tmp/x,source=/var/run/docker.sock,target=/var/run/docker.sock": "IGNORED"
-`,
-			expectedErr: errors.New("parsing batch spec: hooks.onCIFailure step 1 files target path contains invalid characters"),
-		},
 		{
 			name:         "mount path dot-dot traversal",
 			batchSpecDir: tempDir,