fix: remove unused _writableRoots arg to exec() function (#762)

I suspect this was done originally so that `execForSandbox()` had a
consistent signature for both the `SandboxType.NONE` and
`SandboxType.MACOS_SEATBELT` cases, but that is not really necessary and
turns out to make the upcoming Landlock support a bit more complicated
to implement, so I had Codex remove it and clean up the call sites.

codex-cli/tests/raw-exec-process-group.test.ts

@@ -33,7 +33,7 @@ describe("rawExec – abort kills entire process group", () => {
     //  - prints the PID of the `sleep`
     //  - waits for `sleep` to exit
     const { stdout, exitCode } = await (async () => {
-      const p = rawExec(cmd, {}, [], abortController.signal);
+      const p = rawExec(cmd, {}, abortController.signal);
 
       // Give Bash a tiny bit of time to start and print the PID.
       await new Promise((r) => setTimeout(r, 100));