feat(payments-next): Strapi clean up - remove unused fields

Because:

* We didn't ed up using the advertisedSavings, ctaMessage, and modalHeading2 fields for the Cancel Interstitial Offer.

This commit:

* Removes those unused fields from the shared-cms library.

Closes #[PAY-3515](https://mozilla-hub.atlassian.net/browse/PAY-3515)

Author: elizabeth-ilina

libs/payments/ui/src/lib/nestapp/validators/DetermineCancellationInterventionActionResult.ts

@@ -107,22 +107,10 @@ export class CmsCancelInterstitialOfferPartialResult {
   @IsOptional()
   upgradeInterval?: string;
 
-  @IsNumber()
-  @IsOptional()
-  advertisedSavings?: number;
-
-  @IsString()
-  @IsOptional()
-  ctaMessage?: string;
-
   @IsString()
   @IsOptional()
   modalHeading1?: string;
 
-  @IsString()
-  @IsOptional()
-  modalHeading2?: string;
-
   @IsString()
   @IsOptional()
   modalMessage?: string;
@@ -155,18 +143,9 @@ export class CmsCancelInterstitialOfferResult {
   @IsString()
   upgradeInterval!: string;
 
-  @IsNumber()
-  advertisedSavings!: number;
-
-  @IsString()
-  ctaMessage!: string;
-
   @IsString()
   modalHeading1!: string;
 
-  @IsString()
-  modalHeading2!: string;
-
   @IsString()
   modalMessage!: string;