MDL-55528 fileconverter_unoconv: Move env check langstring to core

AMOS BEGIN
 MOV [unoconvwarning,fileconverter_unoconv],[unoconvwarning,admin]
AMOS END

files/converter/unoconv/lang/en/fileconverter_unoconv.php

@@ -36,4 +36,3 @@ $string['test_unoconvnotestfile'] = 'The test document to be coverted into a PDF
 $string['test_unoconvnotexecutable'] = 'The unoconv path points to a file that is not executable';
 $string['test_unoconvok'] = 'The unoconv path appears to be properly configured.';
 $string['test_unoconvversionnotsupported'] = 'The version of unoconv you have installed is not supported.';
-$string['unoconvwarning'] = 'The version of unoconv you have installed is not supported.';

lang/en/admin.php

@@ -1244,3 +1244,4 @@ $string['mobile'] = 'Mobile';
 // Deprecated since Moodle 3.3.
 $string['loginpasswordautocomplete'] = 'Prevent password autocompletion on login form';
 $string['loginpasswordautocomplete_help'] = 'If enabled, users are not allowed to save their account password in their browser.';
+$string['unoconvwarning'] = 'The version of unoconv you have installed is not supported.';