Remove the outdated script for swapping the index files

[froschdesign]

The script to rewrite the index files (swap_index.php) can be removed because the template itself supports a configuration for a custom homepage: show_special_homepage.

Example

extra:
    show_special_homepage: true

https://github.com/zendframework/zend-component-installer/blob/63fff62b5b6835427853fdcfa2f91d1b87cd4774/mkdocs.yml#L8-L9

Updates on Repositories needed

The following pull requests needs to merged before we can remove the script:

• zendframework/zend-mvc-plugin-identity#14 Done!
• zendframework/zend-expressive#671 Done!
• zendframework/zendframework.github.io#14

https://github.com/search?q=org%3Azendframework+filename%3A.zf-mkdoc-theme-landing&type=Code