GH Actions: test run - remove redundant dependency

The `xdebug` extension is already tagged as needed via the `coverage` setting, no need to add it to the `extensions` list.
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Note: generally speaking, I personally normally don't pass an `extensions` list and allow the `setup-php` action to run with the default extensions, which is sufficient in most cases and would be sufficient here as well.

More than anything, I use the `extensions` key to _disable_ extensions for certain test runs, rather than enable them. Just something to consider.

The below documentation should give more insight.

Refs:
* https://github.com/shivammathur/setup-php/wiki/Php-extensions-loaded-on-ubuntu-18.04
* https://github.com/shivammathur/setup-php#heavy_plus_sign-php-extension-support

.github/workflows/tests.yml

@@ -61,7 +61,7 @@ jobs:
           php-version: ${{ matrix.php }}
           coverage: xdebug
           ini-values: sendmail_path=/usr/sbin/sendmail -t -i, zend.multibyte=1, zend.script_encoding=UTF-8, default_charset=UTF-8, error_reporting=E_ALL, display_errors=On
-          extensions: xdebug, imap, mbstring, intl, ctype, filter, hash
+          extensions: imap, mbstring, intl, ctype, filter, hash
 
       # Install dependencies and handle caching in one go.
       # @link https://github.com/marketplace/actions/install-composer-dependencies