[refactor] Remove redundant code related to text vnodes

[kfule]

Description

This PR removes the following redundant code:

• vnode.text-related code, which has not been used internally since Standardise vnode text representation #2670.
• The toString() calls used to compare old and new strings in updateText. Both strings have already been normalized via String().

npm run test passes as-is.

Motivation and Context

This PR is expected to slightly improve bundle size and performance.
In addition, the vnode.text-related code appears very early in the mithril.js build bundle; removing it also improves readability.

How Has This Been Tested?

npm run test

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• I have added tests to cover my changes.
• All new and existing tests passed.
• My change requires a documentation update, and I've opened a pull request to update it already:
• I have read https://mithril.js.org/contributing.html.