Language\en\Language.php is only used in testing

[sfadschm]

This is not really a bug nor an enhancement, but rather some clean-up.

The file https://github.com/codeigniter4/CodeIgniter4/blob/develop/system/Language/en/Language.php is currently only used in testing here:

CodeIgniter4/tests/system/Language/LanguageTest.php

Lines 129 to 136 in 981383d

	public function testLangAllowsOtherLocales()
	{
	$str1 = lang('Language.languageGetLineInvalidArgumentException', [], 'en');
	$str2 = lang('Language.languageGetLineInvalidArgumentException', [], 'ru');
	$this->assertEquals('Get line must be a string or array of strings.', $str1);
	$this->assertEquals('Whatever this would be, translated', $str2);
	}

To remove it, I see two options:

1. Move the file to https://github.com/codeigniter4/CodeIgniter4/tree/develop/tests/_support/Language/en (just like the ru translation file in the same test case).
2. Remove the file and use a message from a core language file which will stay forever for testing.

In both cases, the en\Language.php file can also be removed in all translations in the translations repo. (cross ref. codeigniter4/translations#148)