Remove unused, unsound method on StrInterner

The string slices returned by `get_ref` can actually be
invalidated by calling `clear`.  Since this method is unused,
it is easiest to simply remove it.

Closes #17181
auto merge of #17221 : bkoropoff/rust/strinterner-unsafe, r=sfackler

The `StrInterner::clear()` method takes self immutably but can invalidate references returned by `StrInterner::get_ref`. Since `get_ref` is unused, just remove it.

Closes #17181