Revert "[flags] Predictable should not imply single-threaded"

This reverts commit 5fd38582.

Reason for revert: Failures on the predictable bot: https://ci.chromium.org/ui/p/v8/builders/ci/V8%20Linux%20-%20predictable/36749/overview

Original change's description:
> [flags] Predictable should not imply single-threaded
>
> The --predictable flag is often used to reproduce issues, and having it
> imply --single-threaded can change decisions like which compiler(s) to
> use. This is because --single-threaded is meant to be set by embedders
> (hence we do our best to support single-threaded execution), whereas
> --predictable is a testing-only flag which should not change semantics
> too much. The fact that --predictable executes everything in a single
> thread is already implied by the PredictablePlatform.
>
> R=​ahaas@chromium.org, machenbach@chromium.org
> CC=​​jkummerow@chromium.org
>
> Change-Id: Ic174dd59dfdbd6aa1a410f983db05db26c944cd5
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2919828
> Commit-Queue: Clemens Backes <clemensb@chromium.org>
> Reviewed-by: Andreas Haas <ahaas@chromium.org>
> Reviewed-by: Michael Achenbach <machenbach@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#74822}

Change-Id: Id312cd2b3a150fa3e61daf6550651dc252264ca2
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2922248
Auto-Submit: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#74828}

src/flags/flag-definitions.h

@@ -2022,6 +2022,7 @@ DEFINE_IMPLICATION(print_all_code, print_regexp_code)
 //
 
 DEFINE_BOOL(predictable, false, "enable predictable mode")
+DEFINE_IMPLICATION(predictable, single_threaded)
 DEFINE_NEG_IMPLICATION(predictable, memory_reducer)
 
 DEFINE_BOOL(predictable_gc_schedule, false,

tools/testrunner/local/variants.py

@@ -56,8 +56,8 @@ INCOMPATIBLE_FLAGS_PER_VARIANT = {
               "--validate-asm", "--sparkplug", "--always-sparkplug"],
   "nooptimization": ["--always-opt"],
   "slow_path": ["--no-force-slow-path"],
-  "stress_concurrent_allocation": ["--single-threaded-gc"],
-  "stress_concurrent_inlining": ["--single-threaded"],
+  "stress_concurrent_allocation": ["--single-threaded-gc", "--predictable"],
+  "stress_concurrent_inlining": ["--single-threaded", "--predictable"],
   "stress": ["--always-opt", "--no-always-opt",
              "--max-inlined-bytecode-size=*",
              "--max-inlined-bytecode-size-cumulative=*", "--stress-inline"],