Remove workspace-hack (#40216)

We've been considering removing workspace-hack for a couple reasons:
- Lukas ran into a situation where its build script seemed to be causing
spurious rebuilds. This seems more likely to be a cargo bug than an
issue with workspace-hack itself (given that it has an empty build
script), but we don't necessarily want to take the time to hunt that
down right now.
- Marshall mentioned hakari interacts poorly with automated crate
updates (in our case provided by rennovate) because you'd need to have
`cargo hakari generate && cargo hakari manage-deps` after their changes
and we prefer to not have actions that make commits.

Currently removing workspace-hack causes our workspace to grow from
~1700 to ~2000 crates being built (depending on platform), which is
mainly a problem when you're building the whole workspace or running
tests across the the normal and remote binaries (which is where
feature-unification nets us the most sharing). It doesn't impact
incremental times noticeably when you're just iterating on `-p zed`, and
we'll hopefully get these savings back in the future when
rust-lang/cargo#14774 (which re-implements the functionality of hakari)
is finished.

Release Notes:

- N/A

tooling/xtask/src/tasks/package_conformity.rs

@@ -38,11 +38,6 @@ pub fn run_package_conformity(_args: PackageConformityArgs) -> Result<()> {
             continue;
         }
 
-        // Ignore `workspace-hack`, as it produces a lot of false positives.
-        if package.name == "workspace-hack" {
-            continue;
-        }
-
         for dependencies in [
             &cargo_toml.dependencies,
             &cargo_toml.dev_dependencies,