gpg: drop import-clean from default keyserver import options

* g10/gpg.c (main): drop IMPORT_CLEAN from the
default opt.keyserver_options.import_options
* doc/gpg.texi: reflect this change in the documentation

Given that SELF_SIGS_ONLY is already set, it's not clear what
additional benefit IMPORT_CLEAN provides. Furthermore, IMPORT_CLEAN
means that receiving an OpenPGP certificate from a keyserver will
potentially delete data that is otherwise held in the local keyring,
which is surprising to users who expect retrieval from the keyservers
to be purely additive.

• GnuPG-Bug-Id: T4628
• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>