Remove toStringTag declaration for type compatibility

- Allows Decimal to be type compatible with multiple Decimal.js imports
- Resolves: https://github.com/prisma/prisma/issues/16397

decimal.d.ts

@@ -56,7 +56,6 @@ export declare class Decimal {
   readonly d: number[];
   readonly e: number;
   readonly s: number;
-  private readonly toStringTag: string;
 
   constructor(n: Decimal.Value);
 

decimal.global.d.ts

@@ -77,7 +77,6 @@ export declare class Decimal {
   readonly d: number[];
   readonly e: number;
   readonly s: number;
-  private readonly toStringTag: string;
 
   constructor(n: DecimalValue);