Remove serde rc feature

[rillian]

Description

One of our rust dependencies is enabling the rc feature of the rust serde crate, which isn't recommended since it's easy to misuse. I'd like to track down who needs it and why, in case we can remove it, especially since we're moving to a static build description.

Steps to Reproduce

1. cd build/rust
2. cargo metadata | jq . | less
3. search for "name": "serde"

Actual result:

features array includes rc

Expected result:

Reproduces how often:

Brave version (brave://version info)

True as of 1.55.15.

[rillian]

This is coming from adblock-rust

[antonok-edm]

@rillian the code predates me, but it's from here - NetworkFilterList uses a map from tokens to filters, where each filter can be pointed to by potentially multiple tokens.

As you note, it's easy to misuse, and indeed it looks like it was misused 😅. Serializing and deserializing a DAT with filters pointed to by multiple tokens would duplicate the tokens.

We no longer use the DAT format in brave-core for several reasons, one of which is that it generally ends up being larger than the plaintext lists (maybe this misuse has something to do with that!). I don't plan on maintaining the DAT handling code further; removing it altogether would be a simple solution. Unfortunately, we still need it on iOS until #36035 is completed and enough clients have updated to a version that includes that change.