fix(entity): remove incorrect ComparerStr type (#2584)

BREAKING CHANGE:

The compare function is used in two places, neither of which expect it to be able to return a string:
The first caller is the Array prototype sort function, and there it "should return a negative, zero, or positive value, depending on the arguments".
The second caller does a numerical comparison with the result.

Even though an id can be a string, the result of a comparison shouldn't be.

BEFORE:

The sortComparer types allow for a string to be returned

AFTER:

The sortComparer types only allow a number to be returned

modules/data/spec/entity-metadata/entity-definition.spec.ts

@@ -10,7 +10,7 @@ interface NonIdClass {
   something: any;
 }
 
-const sorter = <T>(a: T, b: T) => 'foo';
+const sorter = <T>(a: T, b: T) => 1;
 
 const filter = <T>(entities: T[], pattern?: any) => entities;
 

modules/entity/src/models.ts

@@ -1,7 +1,4 @@
-export type ComparerStr<T> = (a: T, b: T) => string;
-export type ComparerNum<T> = (a: T, b: T) => number;
-
-export type Comparer<T> = ComparerNum<T> | ComparerStr<T>;
+export type Comparer<T> = (a: T, b: T) => number;
 
 export type IdSelectorStr<T> = (model: T) => string;
 export type IdSelectorNum<T> = (model: T) => number;