Issue:
[REFACTOR] remove unnecessary function from Refunds Validate Flow

### Bug Description
Remove this function `validate_uniqueness_of_refund_id_against_merchant_id`.
Not required as (refund_id, merchant_id ) unique btree, just insert. if insert fails it is not valid .

### Expected Behavior

Not required as (refund_id, merchant_id ) unique btree, just insert. if insert fails it is not valid .

### Actual Behavior

Does extra DB calls

### Steps To Reproduce

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### Context For The Bug

_No response_

### Environment

Are you using hyperswitch hosted version? Yes/No
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If not (or if building/running locally), please provide the following details:
1. Operating System or Linux distribution:
2. Rust version (output of `rustc --version`): ``
3. App version (output of `cargo r --features vergen -- --version`): ``


### Have you spent some time to check if this bug has been raised before?

- [X] I checked and didn't find similar issue

### Have you read the Contributing Guidelines?

- [X] I have read the [Contributing Guidelines](https://github.com/juspay/hyperswitch/blob/main/docs/CONTRIBUTING.md)

### Are you willing to submit a PR?

Yes I am willing to submit a PR!

Resolution:
<think>
PR Context:
- Title: refactor(router): remove unnecessary function from Refunds Validate Flow
- Merged: 2023-10-18T10:44:50Z
- Files changed: 2 files
- Commits: 4 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [x] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->

Refactoring to remove `validate_uniqueness_of_refund_id_against_merchant_id` as it isn't required because of the db being a unique btree. So you are able to try to insert the refund by id and if the insert fails then the refund is not valid 
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** refactor(router): remove unnecessary function from Refunds Validate Flow (#2609)
Co-authored-by: Aar
Files to modify:


**crates/router/src/core/refunds.rs**
  Modify existing code

**crates/router/src/core/refunds/validator.rs**
  Remove:
    - function: pub::validate_uniqueness_of_refund_id_against_merchant_id