Disable per-session thread pool for web (#18480)
### Description
ORT web prefers to use a global thread pool for all inference sessions.
See how OrtCreateSession is implemented in
https://github.com/microsoft/onnxruntime/blob/main/onnxruntime/wasm/api.cc#L183
.
Application code can only the global thread poo. However, internal
testing code still often use per-session threadpool. This pr is to fix
the inconsistency.
### Motivation and Context
Replace PR #18476
