agetty: Remove superfluous fflush()

agetty: Remove superfluous fflush()

eval_issue_file() contains fflush(stdout). It comes from an old code that
used fputs() to write to the console.

In the new code, we write to a temporary memstream, and
fclose(ie->output) fully replaces possible fflush(ie->output) in this
implementation.

The new print_issue_file() does not need it as well, as it uses
unbuffered write_all().

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>