Always parse streams with line buffer (#6532)

Always parse streams with line buffer (#6532)

When cvc5 was compiled in competition mode (but not for the application
track), then it had a special behavior when reading from stdin. When it
received input from stdin, it would read all of stdin and then parse the
input as a string because it assumed that the full input is directly
available on stdin. However, the non-application tracks of SMT-COMP do
not use stdin anymore. They pass a filename to the solver. This special
case is not used as a result. Usually, cvc5 parses from stdin using the
line buffer, so this commit makes it so that this is always the case,
which simplifies the code.