Remove PHP backend details from index.md

Removed PHP backend section from ML backend documentation.

Author: ewallah

docs/apis/plugintypes/mlbackend/index.md

@@ -21,10 +21,6 @@ The communication between machine learning backends and Moodle is through files
 
 ## Backends included in Moodle core
 
-### PHP
-
-The **PHP backend** is the default predictions processor as it is written in PHP and does not have any external dependencies. It is using logistic regression.
-
 ### Python
 
 The **Python backend** requires *python* binary (either python 2 or python 3) and [moodlemlbackend python package](https://pypi.python.org/pypi?name=moodlemlbackend&version=0.0.5&:action=display) which is maintained by Moodle HQ.