TTCHeader::compute_version should be removed or fixed

either we don't compile this structure, in which case we don't need this annotation or to otherwise be generating compile code for this type, or we _do_ want to compile this structure, in which case this fn should be implemented.

> huh this panic should've been removed ages ago, even if we _aren't_ compiling this? will open an issue..
>

_Originally posted by @cmyr in [#1942](https://github.com/googlefonts/fontations/pull/1942/changes#r3462490330)_

Start by locating the TTCHeader::compute_version entry point and review the context from pull request #1942. Determine whether TTCHeader is compiled; done means either removing the unnecessary annotation and generated code or implementing the function, with the panic no longer remaining.

Written by the indexing model from the issue text.