Removing `stub_sandbox` causes some issues

[TheEvilSkeleton]

Removing stub_sandbox in com.microsoft.Edge causes some issues with permissions. Removing it was stated in flathub/com.microsoft.Edge#32 (review) that we could remove it.

This is what I get when I remove it:

❯ flatpak run app/com.microsoft.Edge/x86_64/master
[2 preload-host-spawn-strategy] Running: /app/bin/zypak-helper child - /app/extra/msedge --type=zygote --enable-crashpad --crashpad-handler-pid=20 --enable-crash-reporter=,dev
The setuid sandbox is not running as root. Common causes:
  * An unprivileged process using ptrace on it, like a debugger.
  * A parent process set prctl(PR_SET_NO_NEW_PRIVS, ...)
Failed to move to new namespace: PID namespaces supported, Network namespace supported, but failed: errno = Operation not permitted