gpg: Fix a potential loss of key sigs during import with self-sigs-only.

* g10/import.c (import_one_real): Don't do the final clean in the
merge case.

This fixes a regression introduced with self-sigs-only.

(cherry picked from commit 6701a38f8e4a35ba715ad37743b8505bfd089541)

• GnuPG-bug-id: T4628
• Signed-off-by: Werner Koch <wk@gnupg.org>