gfs2-utils build: Remove superfluous passive header checks
    
    The configure script contained passive header and feature checks which
    set variables depending on what they find, to be used to make code
    portable, but the code does not use those checks. It also duplicated
    some checks for standard headers which autoconf already does itself.
    Remove the unneeded checks and speed up the configure script a bit.
    
    Also remove some checks for headers which are no longer required (since
    the cluster.git split) and remove some unused #includes which were
    discovered along the way.
    
    Signed-off-by: Andrew Price <anprice@redhat.com>