fix: shadow blocks with variable fields are allowed to be deserialized from XML

[ebrahim95]

The basics

• I validated my changes

The details

Resolves

Enabling the ability for shadow blocks to be de-serialized from XML.

Fixes #7779

Proposed Changes

The PR deletes

    // Ensure this block doesn't have any variable inputs.
    if (block.getVarModels().length) {
      throw TypeError('Shadow blocks cannot have variable references.');
    }

in the core/xml.ts file.

Reason for Changes

There is no reason to disallow serialization and also the JSON serialization system does not have this constraint.

Test Coverage

I tested the results by trying to reproduce the error.
The error was fixed as it did not appear in the console.

Documentation

Additional Information

[BeksOmega]

This looks great! Thank you for the fix @ebrahim95 =)