test: remove redundant common.mustCall

All calls verify the cb is called, thus using `common.mustCall` is
not required inside of this function.

PR-URL: nodejs#26738
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

test/parallel/test-fs-readfile-error.js

@@ -38,12 +38,12 @@ function test(env, cb) {
   const filename = fixtures.path('test-fs-readfile-error.js');
   const execPath = `"${process.execPath}" "${filename}"`;
   const options = { env: Object.assign({}, process.env, env) };
-  exec(execPath, options, common.mustCall((err, stdout, stderr) => {
+  exec(execPath, options, (err, stdout, stderr) => {
     assert(err);
     assert.strictEqual(stdout, '');
     assert.notStrictEqual(stderr, '');
     cb(String(stderr));
-  }));
+  });
 }
 
 test({ NODE_DEBUG: '' }, common.mustCall((data) => {