Fixed auto populate of contact refrence field on membership signup form

[pradpnayak]

Overview

A brief description of the pull request. Try to keep it non-technical.

Before

After

Technical Details

The code still used old method of setting contact reference field by default. When contact reference field was changed in core to use Select2 i guess we missed to remove the code from here.

[civibot]

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[adevapp]

The patch worked for me. I was able to signup and then renew (where the contact reference field was already present) a membership without error.

[pradpnayak]

@adam-devapp thanks for QA'ing.

[colemanw]

Code makes sense. Merging based on code review & user testing.