fix: delete contact by prefix

[iFergal]

The endpoint on KERIA doesn't return a JSON response, so deleting via contact is throwing an error:

SyntaxError: Unexpected end of JSON input
  at JSON.parse (<anonymous>)

I can add integration tests to cover contacts if we think it's worthwhile but I'd need to find time.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.51%. Comparing base (6113497) to head (eb6f089).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #264      +/-   ##
==========================================
- Coverage   83.51%   83.51%   -0.01%     
==========================================
  Files          46       46              
  Lines        4210     4209       -1     
  Branches     1047     1047              
==========================================
- Hits         3516     3515       -1     
+ Misses        690      666      -24     
- Partials        4       28      +24     

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[lenkan]

LGTM