Ticket 49237: Drop support for libdb older than 4.7
    
    Bug Description:
    389-ds currently supports many libdb versions which are already burried
    for long time on cemetery for old code. Practically we can assume that
    noone uses libdb older than 4.7 (which is present in RHEL 6). This change
    makes various libdb related code more readable and understandable.
    
    Remove unused parameter from back-ldbm/dblayer.c
    
    https://pagure.io/389-ds-base/issue/49237
    
    Author: Adam Tkac <vonsch@gmail.com>
    
    Review by: wibrown, mreynolds (Thanks for the patch!)