Drop OrdinaryDiffEqTsit5 test dependency and unused solver helpers (#342)

## Summary

- Removes `OrdinaryDiffEqTsit5` as a test dependency.
- Deletes `src/solvers/solver_utils.jl` and
`test/ITensorNetworksTestSolversUtils/`, all dead code.

## Why this is non-breaking

`src/solvers/solver_utils.jl` defined `TimeDependentSum`, `ScaledSum`,
`to_vec`, and `cache_operator_to_disk`, but the file is never `include`d
from `ITensorNetworks.jl` — none of these names are part of the loaded
package and no other `src/` file references them. The only consumer was
`test/ITensorNetworksTestSolversUtils/`, which itself is never loaded by
`runtests.jl` or any `test_*.jl` file (`ITensorPkgSkeleton.runtests`
only auto-loads files starting with `test_`). The whole chain is dead
code, so a patch bump (`0.19.1 -> 0.19.2`) is appropriate.

---------

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

Author: mtfishman

Project.toml

@@ -1,6 +1,6 @@
 name = "ITensorNetworks"
 uuid = "2919e153-833c-4bdc-8836-1ea460a35fc7"
-version = "0.19.2"
+version = "0.19.3"
 authors = ["Matthew Fishman <mfishman@flatironinstitute.org>, Joseph Tindall <jtindall@flatironinstitute.org> and contributors"]
 
 [workspace]

src/solvers/solver_utils.jl

@@ -20,7 +20,6 @@ NDTensors = "23ae76d9-e61a-49c4-8f12-3f1a16adf9cf"
 NamedGraphs = "678767b0-92e7-4007-89e4-4527a8725b19"
 OMEinsumContractionOrders = "6f22d1fd-8eed-4bb7-9776-e7d684900715"
 Observers = "338f10d5-c7f1-4033-a7d1-f9dec39bcaa0"
-OrdinaryDiffEqTsit5 = "b1df2697-797e-41e3-8120-5422d3b24e4a"
 Pkg = "44cfe95a-1eb2-52ea-b672-e2afdf69b78f"
 Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"
 SafeTestsets = "1bc83da4-3b8d-516f-aca4-4fe02f6d838f"
@@ -57,7 +56,6 @@ NDTensors = "0.3, 0.4"
 NamedGraphs = "0.8.2"
 OMEinsumContractionOrders = "0.9.5, 1"
 Observers = "0.2.5"
-OrdinaryDiffEqTsit5 = "1.1, 2"
 Pkg = "1.10"
 Random = "1.10"
 SafeTestsets = "0.1"