Revert "Prevent nonsensical comparations between OUString and nullptr_t"

This reverts commit 2e3f5c8d, some GCC
have trouble with deleted friend functions; need to fix that properly.

include/rtl/ustring.hxx

@@ -1588,17 +1588,6 @@ public:
     friend bool     operator != ( const sal_Unicode * pStr1,    const OUString& rStr2 )
                         { return !(operator == ( pStr1, rStr2 )); }
 
-#if defined LIBO_INTERNAL_ONLY
-    // Prevent the above overloads from being selected for nonsensical code like
-    //
-    //   if (ouIdAttr == nullptr)
-    //
-    friend bool operator ==(OUString const &, std::nullptr_t) = delete;
-    friend bool operator ==(std::nullptr_t, OUString const &) = delete;
-    friend bool operator !=(OUString const &, std::nullptr_t) = delete;
-    friend bool operator !=(std::nullptr_t, OUString const &) = delete;
-#endif
-
     friend bool     operator <  ( const OUString& rStr1,    const OUString& rStr2 )
                         { return rStr1.compareTo( rStr2 ) < 0; }
     friend bool     operator >  ( const OUString& rStr1,    const OUString& rStr2 )