g10/decrypt-data: disable output estream buffering to reduce overhead

* g10/decrypt-data.c (decrypt_data): Disable estream buffering for
output file.

Here estream is filled with iobuf_copy which already uses large buffers
so additional buffering in estream was just adding memory copy overhead.

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