gpg: Clarify a call of ask_for_detached_datafile.

* g10/mainproc.c (proc_tree): Call ask_for_detached_datafile
with MD2=NULL.

Here, c->mfx.md2 is always NULL, in fact. But, text-wise
(when searching the use of "mfx.md2"), before the change, it used
mfx.md2, which is irrelevant in the handling of PKT_ONEPASS_SIG.

Note that: One-Pass Signature is not available in PGP2.

This fix removes (text-wise) unmatch of the calls of functions
hash_datafile_by_fd hash_datafiles, and ask_for_detached_datafile.

• Fixes-commit: 88a916cdd40e43312ffcde6bb1c157fe1c122f74
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>