remove workaround for controlling terminal behavior in repl_cmd

[vtjnash]

This was originally added as a workaround for the behavior of sh to try to become the terminal process group leader, but no longer relevant since #25006 removed that flag again:

julia> run(detach(`bash -i -c "sleep 0"`))
bash: cannot set terminal process group (-1): Inappropriate ioctl for device
bash: no job control in this shell
Process(`bash -i -c 'sleep 0'`, ProcessExited(0))

Julia recieves SIGTTOU when a process like "sh -i -c" exits (at least for bash and zsh, but not dash, ksh, or csh), since "sh -i" sometimes takes over the controlling terminal, causing julia to stop once the bash process exits. This can be quite annoying, but julia goes through some pains (in libuv) to ensure it doesn't steal the controlling terminal from the parent, and apparently bash is not so careful about it. However, since PR #25006, julia hasn't needed this workaround for this issue, so we can now follow the intended behavior when the child is in the same session group and steals the controlling terminal from the parent. Even if such behavior seems odd, this seems to be the intended behavior per posix design implementation that it gets SIGTTOU when julia later tries to change mode (from cooked -> raw after printing the prompt): http://curiousthing.org/sigttin-sigttou-deep-dive-linux.

$ ./julia -q
shell> bash -i -c "sleep 1"

[1]+  Stopped                 ./julia

julia> run(`zsh -i -c "sleep 0"`)
Process(`zsh -i -c 'sleep 0'`, ProcessExited(0))

julia>
[1]+  Stopped                 ./julia

[vtjnash]

For background on why this is a useful signal and behavior and should not be just blocked, see nodejs/node#35536

[vtjnash]

According to glibc, there's a half page of code for how to correctly implement a REPL mode change call: https://www.gnu.org/software/libc/manual/html_node/Initializing-the-Shell.html