MM-23817: Use auto-vendoring (#14231)

With Go 1.14 (https://tip.golang.org/doc/go1.14#go-command),
it now automatically checks for the existence of a vendor directory.

So there is no need to explicitly set the `-mod=vendor` flag.

utils/test_files_compiler.go

@@ -33,7 +33,7 @@ func CompileGo(t *testing.T, sourceCode, outputPath string) {
 	serverPath := filepath.Dir(filepath.Dir(sourceFile))
 
 	out := &bytes.Buffer{}
-	cmd := exec.Command("go", "build", "-mod=vendor", "-o", outputPath, main)
+	cmd := exec.Command("go", "build", "-o", outputPath, main)
 	cmd.Dir = serverPath
 	cmd.Stdout = out
 	cmd.Stderr = out