Remove unused `$LANG` global variable

[cedric-anne]

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	-

This variable is not used by GLPI.

I did not find any usage in plugins that would be affected by this removal. Indeed, the following snippets will still work:

// a simple declaration
global $LANG;
$LANG['pluginkey']['xxx'] = 'value';

// will anyway trigger a warning if the corresponding entry is not defined
global $LANG
echo $LANG['pluginkey']['xxx'];

// a safe usage
global $LANG;
if (isset($LANG['genericobject'][$class][0])) {
    return $LANG['genericobject'][$class][0];
}

[orthagh]

First snippet doesn't trigger a warning about an uninitialized array ?

[cedric-anne]

First snippet doesn't trigger a warning about an uninitialized array ?

No, PHP consider it as a valid variable initialization.