## Problem
The [master WebAssembly
Build](https://github.com/Z3Prover/z3/actions/runs/28306680131) fails
with:
```
../src/solver/parallel_tactical.cpp:59:9: error: redefinition of 'mk_parallel_tactic'
59 | tactic* mk_parallel_tactic(solver* s, params_ref const& /* p */) {
../src/solver/parallel_tactical.cpp:55:9: note: previous definition is here
```
## Cause
Commit 7564ccc3f (an unrelated lar_solver change) accidentally renamed
the dead `mk_parallel_tactic2` stub to `mk_parallel_tactic`, leaving two
identical definitions inside the `#ifdef SINGLE_THREAD` block. The WASM
build defines `SINGLE_THREAD`, so it hits the redefinition.
## Fix
`mk_parallel_tactic2` and its `non_parallel_tactic2` class were never
referenced anywhere. This removes the dead stub and orphaned class,
keeping the single `mk_parallel_tactic` that degrades to
`mk_solver2tactic(s)` in single-threaded mode (added in #9977).
Verified both `SINGLE_THREAD` and multi-threaded paths pass
`-fsyntax-only`.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
f07acb459f