kbx: Unify the fingerprint search modes.

* kbx/keybox-search-desc.h (KEYDB_SEARCH_MODE_FPR16)
(KEYDB_SEARCH_MODE_FPR20, KEYDB_SEARCH_MODE_FPR32): Remove.  Switch
all users to KEYDB_SEARCH_MODE_FPR along with the fprlen value.

These search modes were added over time and there has until recently
be no incentive to remove the cruft. With the change for v5 keys I
finally went over all places and allowed the generic fingerprint mode
along with a given length of the fingerprint at all places.
Consequently the other modes can now be removed.

• Signed-off-by: Werner Koch <wk@gnupg.org>