terminal-util: remove unnecessary check of result of isatty() (#4000)

terminal-util: remove unnecessary check of result of isatty() (#4000)

After the call of the isatty() we check its result twice in the
open_terminal(). There are no sense to check result of isatty() that
it is less than zero and return -errno, because as described in
documentation:

isatty() returns 1 if fd is an open file descriptor referring to a
terminal;  otherwise 0 is returned, and errno is set to indicate the
error.

So it can't be less than zero.