[2.3][Process] Remove unreachable code + avoid skipping tests in sigchild environment

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

As mentioned by @Tobion in https://github.com/symfony/symfony/pull/10480#issuecomment-38002910, I removed the dead code. I also fixed/updated the test suite on PHP compiled with `--enable-sigchild`.