Biblatex-chicago omit prefix in postnote

biblatex-chicago is an extremely complicated style bundle, because it tries to implement the requirements of the Chicago Manual of Style as closely as possible with biblatex. As such it can be fairly difficult to modify biblatex-chicago to deviate from CMoS requirements. Some changes need lots of code, others have the potential to break desired behaviour in unexpected places.

As such I cannot guarantee that the following code does not have unintended side-effects; doubly so because there is no example document (MWE: I've just been asked to write a minimal working example (MWE), what is that?) in the question.

renewrobustcmd*{mkjuridprefix}[1]{#1}%

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Your DeclareFieldFormat{postnote} did not work, because biblatex-chicago uses type-specific formats to typeset the postnote field, but DeclareFieldFormat{postnote} only changes the non-type-specific format. You could have used the starred version DeclareFieldFormat*{postnote}

DeclareFieldFormat*{postnote}{% Changed for page compression option
  ifboolexpr{%
    togl {cms@comprange}%
    and
    test {ifpages{#1}}%
  }%
  {iffieldundef{pagination}%
    {mkcomprange{#1}}%
    {mkcomprange[{mkpageprefix[pagination]}]{#1}}}%
  {iffieldundef{pagination}%
    {#1}%
    {mkpageprefix[pagination]{#1}}}}%

to get rid of all type-specific formats first. (For more examples on type-specific format, see Remove Quotation Marks from Style)