Fix APIv5 attempting to validate Delete-type DSRs as v4 structures

[ocket8888]

The APIv5 handler for creating Delivery Service Requests to delete a Delivery Service currently attempts to cast the structure to a v4 DSR struct, which results in a false validation failure. This PR fixes that.

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Which Traffic Control components are affected by this PR?

• Traffic Ops

What is the best way to verify this PR?

1. Make a DS
2. Create a DSR to delete that DS and make sure that succeeds.

If this is a bugfix, which Traffic Control versions contained the bug?

• master

PR submission checklist

• This PR has tests
• This PR doesn't need documentation
• This PR doesn't need a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY

[codecov]

Codecov Report

Merging #7256 (0e2f841) into master (20882f7) will not change coverage.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master    #7256   +/-   ##
=========================================
  Coverage     28.29%   28.29%           
  Complexity       98       98           
=========================================
  Files           617      617           
  Lines         69363    69363           
  Branches         90       90           
=========================================
  Hits          19626    19626           
  Misses        47925    47925           
  Partials       1812     1812           

Flag	Coverage Δ
golib_unit	52.87% <ø> (ø)
grove_unit	4.60% <ø> (ø)
t3c_generate_unit	24.96% <ø> (ø)
traffic_monitor_unit	20.43% <ø> (ø)
traffic_ops_integration	69.41% <ø> (ø)
traffic_ops_unit	19.75% <0.00%> (ø)
traffic_stats_unit	10.41% <ø> (ø)
v3	57.68% <ø> (ø)
v4	79.65% <ø> (ø)
v5	78.53% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...fic_ops_golang/deliveryservice/request/validate.go	0.00% <0.00%> (ø)

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[rimashah25]

LGTM. I am able to delete a DS via DSR and don't see the previous error original: expected a Delivery Service, got *tc.DeliveryServiceV50.