templates: Remove wit-bindgen-backport dependency

[lann]

This is the last vestage of "old" wit-bindgen in spin; app usage of the old ABI is now handled by the spin-componentize adaptation.

After some discussion, I think one of the following should be done:

• Remove the custom filters feature; the only known usage of it is this simple filter which could be moved into a regular filter impl in spin-templates.

• Commit the wit-bindgen-generated source for the custom-filter.wit rather than using the wit-bindgen macro. Breaking changes in the wasmtime APIs that this generated code uses should be rare but would require manual fixes to that code. Guest code would be stuck with old wit-bindgen versions, but that isn't necessarily a major problem for this use.

• Convert custom filters to use the component model. I think as long as the custom filter "world" doesn't include WASI this might be fine.

[lann]

I think we've settled on the breaking change of removing the functionality for 2.0.