Remove unused library setting circ.holds.target_holds_by_org_unit_weight

[Galen Charlton]

The library setting circ.holds.target_holds_by_org_unit_weight was added in commit 9ee5a7538c96291a71d1c but was never used.

It doesn't look like it would ever serve a purpose, as weight-based targeting is enabled by setting the circ.holds.org_unit_target_weight org unit setting to a value other than one, so circ.holds.target_holds_by_org_unit_weight should just be removed as a bit of tidying.

[Terran McCanna (tmccanna)]

Noting that this setting is also in the documentation and should be removed from there - https://docs.evergreen-ils.org/docs/latest/admin/librarysettings.html