Trailing underscore in fragment spread sub types

**Describe the bug**
The generated sub types for fragment spreads end with a trailing underscore, e.g. `MandateFragment_DigitalMandate_`. The expected behaviour is generating the name without the trailing underscore, e.g. `MandateFragment_DigitalMandate`.

**To Reproduce**
Steps to reproduce the behavior:

https://codesandbox.io/s/graphql-codegen-fragment-spread-bug-1yovy?file=/types.ts

1. My GraphQL schema:

```graphql
type Query {
mandates: [Mandate!]!
}

interface Mandate {
id: ID!
reason: String!
}

type DigitalMandate implements Mandate {
id: ID!
reason: String!
code: String!
}

type PaperMandate implements Mandate {
id: ID!
reason: String!
file: String!
}
```

2. My GraphQL operations:

```graphql
fragment MandateFragment on Mandate {
id
reason

...DigitalMandateFragment
...PaperMandateFragment
}

fragment DigitalMandateFragment on DigitalMandate {
code
}

fragment PaperMandateFragment on PaperMandate {
file
}

query GetMandates {
mandates {
...MandateFragment
}
}
```

3. My `codegen.yml` config file:

```yml
schema: schema.graphql
documents: document.graphql
generates:
types.ts:
plugins:
- typescript
- typescript-operations
config:
dedupeOperationSuffix: true
exportFragmentSpreadSubTypes: true
namingConvention: keep
```

**Expected behavior**
The name should not end with a trailing underscore, so `MandateFragment_DigitalMandate` instead of `MandateFragment_DigitalMandate_`.

**Environment:**

- OS: Ubuntu 18.04
- `@graphql-codegen/...`: 1.15.4
- NodeJS: 14.4.0 (or 10.20.1 in the CodeSandbox)

**Additional context**
-