g10/mainproc: disable hash contexts when --skip-verify is used

* g10/mainproc.c (proc_plaintext): Do not enable hash contexts when
opt.skip_verify is set.

(cherry picked from commit 73e74de0e33bbb76300f96a4174024779047df06)

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