Revert introduction of [workspace.dependencies].

This was done in #145740 and #145947. It is causing problems for people
using r-a on anything that uses the rustc-dev rustup package, e.g. Miri,
clippy.

This repository has lots of submodules and subtrees and various
different projects are carved out of pieces of it. It seems like
`[workspace.dependencies]` will just be more trouble than it's worth.

Cargo.toml

@@ -59,33 +59,6 @@ exclude = [
   "obj",
 ]
 
-[workspace.dependencies]
-# tidy-alphabetical-start
-bitflags = "2.9.3"
-derive-where = "1.6.0"
-either = "1.15.0"
-indexmap = "2.10.0"
-itertools = "0.12.1"
-# FIXME: Remove this pin once this rustix issue is resolved
-# https://github.com/bytecodealliance/rustix/issues/1496
-libc = "=0.2.174"
-measureme = "12.0.3"
-memchr = "2.7.5"
-odht = { version = "0.3.1", features = ["nightly"] }
-polonius-engine = "0.13.0"
-proc-macro2 = "1.0.101"
-quote = "1.0.40"
-rustc-demangle = "0.1.26"
-rustc-hash = "2.1.1"
-rustc-literal-escaper = "0.0.5"
-rustc_apfloat = "0.2.3"
-scoped-tls = "1.0.1"
-serde_json = "1.0.142"
-tempfile = "3.20.0"
-thin-vec = "0.2.14"
-tracing = "0.1.37"
-# tidy-alphabetical-end
-
 [profile.release.package.rustc_thread_pool]
 # The rustc fork of Rayon has deadlock detection code which intermittently
 # causes overflows in the CI (see https://github.com/rust-lang/rust/issues/90227)