refactor(symfony-process): drop the --enable-sigchild workarounds

isSigchildEnabled() detects a PHP built with --enable-sigchild, where PHP
reaps children itself and proc_get_status()/proc_terminate() stop reporting
or reaching them. Shirabe spawns child processes from Rust, so that build
option cannot affect them and every sigchild branch was unreachable.

Removing the branches retires the state that existed only to feed them:
fallbackStatus (written by the fourth pipe and by doSignal, read only by
the sigchild merge in updateStatus) and useFileHandles (whose sole reader
was the sigchild condition). shirabe-php-shim loses phpinfo(),
INFO_GENERAL and posix_kill(), which had no other callers.

DiagnoseCommand keeps its --enable-sigchild warning: it reports on the
user's PHP installation, not on how Shirabe runs processes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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