Drop unused DataStructures dependency

`DataStructures` was narrowed to `"0.18"` in #328 because the `EinExprs`
weakdep pinned it that way. The `EinExprs` dependency was removed in
#327, but the narrowed `DataStructures` compat was never reverted (and
`DataStructures` itself is no longer `using`'d anywhere in `src/`,
`ext/`, or `test/`). Drop the entry entirely from `[deps]` and `[compat]`.

This also unblocks `OrdinaryDiffEqTsit5 = "1.1, 2"` resolution: the
v2 chain pulls in `OrdinaryDiffEqCore` 4, which requires
`DataStructures` 1.x-3.x and was previously incompatible with the 0.18 pin.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: mtfishman

Project.toml

@@ -14,7 +14,6 @@ Combinatorics = "861a8166-3701-5b0c-9a16-15d98fcdc6aa"
 Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"
 ConstructionBase = "187b0558-2788-49d3-abe0-74a17ed4e7c9"
 DataGraphs = "b5a273c3-7e6c-41f6-98bd-8d7f1525a36a"
-DataStructures = "864edb3b-99cc-5e75-8d2d-829cb0a9cfe8"
 Dictionaries = "85a47980-9c8c-11e8-2b9f-f7ca1fa99fb4"
 Distributions = "31c24e10-a181-5473-b8eb-7969acd0382f"
 DocStringExtensions = "ffbed154-4ef7-542d-bbb7-c09d3a79fcae"
@@ -56,7 +55,6 @@ Combinatorics = "1"
 Compat = "3, 4"
 ConstructionBase = "1.6"
 DataGraphs = "0.2.13"
-DataStructures = "0.18"
 Dictionaries = "0.4"
 Distributions = "0.25.86"
 DocStringExtensions = "0.9"