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.