docs: remove environment-specific configuration files

There is no such a thing.

Author: kenjis

user_guide_src/source/general/environments.rst

@@ -103,11 +103,3 @@ Conversely, setting the constant to 'production' will disable all error
 output. Disabling error reporting in production is a
 :doc:`good security practice </concepts/security>`.
 
-Configuration Files
--------------------
-
-Optionally, you can have CodeIgniter load environment-specific
-configuration files. This may be useful for managing things like
-differing API keys across multiple environments. This is described in
-more detail in the Handling Different Environments section of the
-:doc:`Working with Configuration Files </general/configuration>` documentation.