Remove explorer's proto fuzzer and proto dependencies

[jonmeow]

As part of migrating to the latest bazel configurations in #4729, I'm running into proto toolchain issues. For example:
"Error: <target @@protobuf+//:cc_toolchain> (rule 'proto_lang_toolchain') doesn't contain declared provider 'ProtoLangToolchainInfo'"

Although we may eventually want more use of proto, right now the only use is for the explorer fuzzer. The explorer codebase is essentially frozen, so continuing to run it isn't gaining us much (in fact, we've already disabled autofuzzing for it).

So, rather than trying to fix the proto setup, this change:

1. Deletes explorer/fuzzing
2. Removes proto portions of testing/fuzzing, which were only in-use by the explorer
3. Removes some ancillary proto support, which would otherwise break from the bazel changes and would be difficult to validate as "still working"

This change is partly isolated in order to make it easier to revive bits of (3).

[zygoloid]

LG. I briefly wondered if it'd be worth converting the existing fuzz corpus to carbon source files and adding it to the toolchain fuzz corpus, but it looks like we already did that :)