[Process] Remove unreachable code + avoid skipping tests in sigchild environment
minor #10492 [2.3][Process] Remove unreachable code + avoid skipping tests in sigchild environment (romainneutron)

This PR was merged into the 2.3 branch.

Discussion
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[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`.

Commits
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d52dd32 [Process] Remove unreachable code + avoid skipping tests in sigchild environment