refactor: remove unnecessary checks

[HcySunYang]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch for v2.x (or to a previous version branch), not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:
Through the processing of the mergeOptions function, the type of vm.$options.data is either a function or undefined. And in the initState function has filtered out the possibility of undefined, so in the initData function vm.$options.data must be a function

[jo0ger]

Hi, I think these checks are necessary.
For example:

const app = new Vue({
    el: '#app',
    data: {
        name: 'Jooger'
    },
    beforeCreate () {
        this.$options.data = {
            name: 'Jooger2'
        }
    }
})

Because beforeCreate is executed before initData, it is necessary to check the data type in initData if I change vm.$option.data in beforeCreate

[posva]

@jo0ger They were reverted