Subzero supported sandboxing for the PNaCl platform. Reactor does not
support sandboxing at the JIT level, and we don't have a need for it
since Chromium provides sandboxing as part of the "GPU process". Thus
we can remove it and reduce code complexity.

Note that Subzero's sandboxing implementation comes at a performance
penalty. Project Bunker provides a better solution for SwiftShader,
which is probably also more secure in light of speculative execution
vulnerabilities.

If we ever do need sandboxing support in Reactor itself (e.g. outside of
SwiftShader, when process isolation is not feasible), it is best to
use an actively developed JIT-compiler where security always takes
priority over peformance, like Chromium's WebAssembly JIT.

Bug: b/179832693
Change-Id: I7364d22183e123c5145caae9f546d3855012d73e
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/55488
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Commit-Queue: Nicolas Capens <nicolascapens@google.com>