GH Actions: remove build with special set of extensions

There was a special build included which is supposed disable Mbstring and record code coverage, but the `extensions` key was missing from the `setup-php` action runner, so Mbstring was never disabled....

Another thing to know about this build is that Mbstring is a required extension for PHPUnit, but it is only really needed when PHPUnit is run with the `--testdox` CLI flag.
In later PHPUnit versions, this requirement is enforced across the board (Composer + PHAR), but that's not yet the case for the PHPUnit 8 PHAR file. However, as we run the tests via a Composer install, that install will still fail on the PHPUnit `mbstring` requirement.

All in all, this build adds nothing and can be removed.

Author: jrfnl

.github/workflows/unit-tests.yml

@@ -32,11 +32,6 @@ jobs:
         coverage: [false]
 
         include:
-          - php: '7.2'
-            dependencies: 'stable'
-            extensions: ':mbstring' # = Disable Mbstring.
-            coverage: true # Make sure coverage is recorded for this too.
-
           # Run code coverage builds against high/low PHP and high/low PHPCS.
           - php: '7.2'
             dependencies: 'stable'