g10/plaintext: disable estream buffering in binary mode

* g10/plaintext.c (handle_plaintext): Disable estream buffering in
binary modes.

Since in binary mode, large buffers are passed from source iobuf to
target estream, extra buffering in estream only causes extra memory
copying and overhead.

• GnuPG-bug-id: T5828
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>