Remove PHP version check from the plugin bootstrap file

[cabrerahector]

Starting version 5.2 WordPress automatically checks whether the site meets plugin's minimum required PHP version or not, see Version 5.2 Release Notes - Plugin Compatibility Checks.

At the time of writing WordPress Popular Posts requires at least WordPress 4.9 to work. Once the plugin bumps its minimum required WordPress version to 5.2 we'll no longer need to do that check in the bootstrap file.

[cabrerahector]

Note: since version 5.8 WordPress no longer checks readme.txt to validate requirements, see Remove parsing of readme.txt files from validate_plugin_requirements().

In short:

• Requires PHP and Requires at least must be defined in the plugin's main PHP file.
• Tested up to should still be defined in readme.txt.