Config: remove code related to the now unsupported PEAR installation

PR 4 dropped support for installing PHPCS via PEAR. This commit removes
code related to a PEAR placeholder called `@data_dir@` as it is not
necessary anymore.

The original version of this code was added in 980c835#diff-c36ecedca179eab0b3cd245e872a96ab26fa08e567437e167f4eda1779c15c89R1370-R1372.
Two methods were added that use the `@data_dir@` placeholder in that
commit: `setConfigData()` and `getAllConfigData()`. A subsequent commit
changed `setConfigData()` to only use the PEAR path for the config file
if the `@data_dir@` placeholder was replaced with something else:
7bb7383.
A later commit did the same for `getAllConfigData()` together with other
unrelated changes: 9392185.

It seems that the PEAR file package.xml was responsible for replacing
`@data_dir@` with an actual path: https://github.com/PHPCSStandards/PHP_CodeSniffer/blob/6d22f38f6c846512d321f3d5ac15c8805c694084/package.xml#L293-L295.

PR 4 removed the `@data_dir@` related code from `setConfigData()`, but
not from `getAllConfigData()`.

Closes 1101

Author: rodrigoprimo

src/Config.php

@@ -1704,11 +1704,6 @@ public static function getAllConfigData()
             $configFile = dirname($path).DIRECTORY_SEPARATOR.'CodeSniffer.conf';
         } else {
             $configFile = dirname(__DIR__).DIRECTORY_SEPARATOR.'CodeSniffer.conf';
-            if (is_file($configFile) === false
-                && strpos('@data_dir@', '@data_dir') === false
-            ) {
-                $configFile = '@data_dir@/PHP_CodeSniffer/CodeSniffer.conf';
-            }
         }
 
         if (is_file($configFile) === false) {