Remove dead code related to tty allocation for ptys. Back in revision 1.17 of

Remove dead code related to tty allocation for ptys. Back in revision 1.17 of
kern/tty_pty.c, allocation of ptys was made dynamic with the introduction of
check_pty(). Every time a new struct pty is allocated its corresponding struct
tty is also allocated. It's therefore no longer necessary to ensure that a pty
has a tty allocated after calling check_pty().

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