Remove payment subgraph ternary from initializeRN.ts in Request Invoicing

[MantisClone]

Problem

The initializeRN.ts file has a long ternary for configuring the payment subgraph URLs that was added in #41

But now that the RN SDK provides paid subgraph URLs hosted by Alchemy by default, we should remove the long ternary.

Definition of Done

• Remove payment subgraph override ternary from initializeRN.ts in Request Invoicing