Remove unnecessary calls to wrap_here and gdb_flush

Various spots in gdb currently know about the wrap buffer, and so are
careful to call wrap_here to be certain that all output has been
flushed.

Now that the pager is just an ordinary stream, this isn't needed, and
a simple call to gdb_flush is enough.

Similarly, there are places where gdb prints to gdb_stderr, but first
flushes gdb_stdout.  stderr_file already flushes gdb_stdout, so these
aren't needed.