SimSYCL is now thread-safe; remove workarounds

[fknorr]

Since celerity/SimSYCL#14 is now merged, we don't need explicit threading workarounds in the backend anymore.

[github-actions]

Check-perf-impact results: (000e2892abb21ddd1ae413a5c86d7d95)

❓ No new benchmark data submitted. ❓
Please re-run the microbenchmarks and include the results if your commit could potentially affect performance.

[github-actions]

⚠️Clang-Tidy found issue(s) with the introduced code (1/1)

[coveralls]

Pull Request Test Coverage Report for Build 12787716791

Details

• 4 of 4(100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.07%) to 95.128%

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Files with Coverage Reduction	New Missed Lines	%
src/backend/sycl_backend.cc	2	89.86%

Totals
Change from base Build 12787655228:	0.07%
Covered Lines:	7085
Relevant Lines:	7186

---

💛 - Coveralls

[github-actions]

⚠️Clang-Tidy found issue(s) with the introduced code (1/1)

[github-actions]

⚠️Clang-Tidy found issue(s) with the introduced code (1/1)

[github-actions]

Check-perf-impact results: (c8fb992b35322012b54e351345fdf71a)

❓ No new benchmark data submitted. ❓
Please re-run the microbenchmarks and include the results if your commit could potentially affect performance.