Bug 11751: remove dead code related to handling members of institution patrons

Bug 11751: remove dead code related to handling members of institution patrons

This patch removes some dead code concerning the handling of patrons
that are members of other, institutional patrons.  This code did not
work; removing it clears the field if somebody wants to do a better
implementation of such relationships between patrons.

This patch:

[1] Removes the memberofinstitution system preference.
[2] Removes the following routines:

    C4::Members::get_institutions()
    C4::Members::add_member_orgs() (and removing this routine
    removes a reference to a borrowers_to_borrowers table that
    does not exist).

There should be no changes whatsoever to system functionality with this
patch (with the trivial exception of the absence of the
memberofinstitution system preference).

Test plan:

[1] Look at the code and use grep, git grep, etc. verify this patch
    does not remove something in use.
[2] Verify that there are no regressions upon adding or editing
    a patron record.
[3] Verify that the memberofinstitution system preference has been
    removed

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>