feat: Replace contentTypeMixin within the component (#9802)

# Pull Request Template

## Description

This PR will completely remove the `contentTypeMixin` and be used in the
component as it is.

Fixes
https://linear.app/chatwoot/issue/CW-3465/rewrite-contenttypemixin-mixin-as-a-helper

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

Take a look at this component
`app/javascript/dashboard/components/widgets/conversation/Message.vue`


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/javascript/shared/mixins/contentTypeMixin.js

@@ -1,9 +0,0 @@
-import { CONTENT_TYPES } from '../constants/contentType';
-
-export default {
-  computed: {
-    isEmailContentType() {
-      return this.contentType === CONTENT_TYPES.INCOMING_EMAIL;
-    },
-  },
-};

app/javascript/shared/mixins/specs/contentTypeMixin.spec.js

@@ -1,32 +0,0 @@
-import { shallowMount } from '@vue/test-utils';
-import contentTypeMixin from '../contentTypeMixin';
-
-describe('contentTypeMixin', () => {
-  it('returns true if contentType is incoming_email', () => {
-    const Component = {
-      render() {},
-      mixins: [contentTypeMixin],
-      computed: {
-        contentType() {
-          return 'incoming_email';
-        },
-      },
-    };
-    const wrapper = shallowMount(Component);
-    expect(wrapper.vm.isEmailContentType).toBe(true);
-  });
-
-  it('returns false if contentType is not incoming_email', () => {
-    const Component = {
-      render() {},
-      mixins: [contentTypeMixin],
-      computed: {
-        contentType() {
-          return 'input_select';
-        },
-      },
-    };
-    const wrapper = shallowMount(Component);
-    expect(wrapper.vm.isEmailContentType).toBe(false);
-  });
-});