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

crates/eval/src/examples/threads/overwrite-file.json

@@ -116,7 +116,7 @@
       ],
       "tool_results": [
         {
-          "content": "[package]\nname = \"language_model_selector\"\nversion = \"0.1.0\"\nedition.workspace = true\npublish.workspace = true\nlicense = \"GPL-3.0-or-later\"\n\n[lints]\nworkspace = true\n\n[lib]\npath = \"src/language_model_selector.rs\"\n\n[dependencies]\ncollections.workspace = true\nfeature_flags.workspace = true\nfuzzy.workspace = true\ngpui.workspace = true\nlanguage_model.workspace = true\nlog.workspace = true\npicker.workspace = true\nproto.workspace = true\nui.workspace = true\nworkspace-hack.workspace = true\nzed_actions.workspace = true\n",
+          "content": "[package]\nname = \"language_model_selector\"\nversion = \"0.1.0\"\nedition.workspace = true\npublish.workspace = true\nlicense = \"GPL-3.0-or-later\"\n\n[lints]\nworkspace = true\n\n[lib]\npath = \"src/language_model_selector.rs\"\n\n[dependencies]\ncollections.workspace = true\nfeature_flags.workspace = true\nfuzzy.workspace = true\ngpui.workspace = true\nlanguage_model.workspace = true\nlog.workspace = true\npicker.workspace = true\nproto.workspace = true\nui.workspace = true\n\nzed_actions.workspace = true\n",
           "is_error": false,
           "output": null,
           "tool_use_id": "toolu_019Je2MLfJhpJr93g5igoRAH"