[lldb] Revert custom __str__ in SBStructuredDataExtensions.i (#156721)

`__str__` was implemented in #155061, however its behavior was limited
to only a some kinds of `SBStructuredData`. That was a breaking change,
and this change removes that implementation of `__str__`, relying on the
existing behavior which calls `GetDescription`.

GitOrigin-RevId: 722339dc0927545834afffd8aca9a75efd450ca9