ldif: removed ldif_fix_string()

WARNING: this will change the default output file
 encoding when importing a LDIF input!

- An LDIF input of UTF-8 entries is base64-encoded (RFC)
 str_parse_line() already supports that pretty well.
- But ldif_fix_string(), present since the initial
 CVS revision of abook, forces the conversion of utf-8
 wide-chars to latin1.
- And that's why iconv exists

=> so let's drop it unless someone complains for a good reason.