Revert "[baseline] Enable concurrent sparkplug"

This reverts commit f01a6098.

Reason for revert: Consistent TSan failures: https://crbug.com/v8/12713

Original change's description:
> [baseline] Enable concurrent sparkplug
>
> Bug: v8:12054
> Change-Id: I1b0ee1a9541a75412b882b259e421aa6e88317da
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3259658
> Auto-Submit: Victor Gomes <victorgomes@chromium.org>
> Reviewed-by: Toon Verwaest <verwaest@chromium.org>
> Commit-Queue: Toon Verwaest <verwaest@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#79485}

Bug: v8:12054
Change-Id: Ide6bbac8c280a1373c5082bbe5356a205f3396c5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3528496
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/main@{#79492}

src/flags/flag-definitions.h

@@ -711,7 +711,7 @@ DEFINE_BOOL(baseline_batch_compilation, true, "batch compile Sparkplug code")
 DEFINE_BOOL_READONLY(concurrent_sparkplug, false,
                      "compile Sparkplug code in a background thread")
 #else
-DEFINE_BOOL(concurrent_sparkplug, true,
+DEFINE_BOOL(concurrent_sparkplug, false,
             "compile Sparkplug code in a background thread")
 DEFINE_WEAK_IMPLICATION(future, concurrent_sparkplug)
 DEFINE_NEG_IMPLICATION(predictable, concurrent_sparkplug)
@@ -719,7 +719,7 @@ DEFINE_NEG_IMPLICATION(single_threaded, concurrent_sparkplug)
 DEFINE_NEG_IMPLICATION(jitless, concurrent_sparkplug)
 #endif
 DEFINE_UINT(
-    concurrent_sparkplug_max_threads, 2,
+    concurrent_sparkplug_max_threads, 0,
     "max number of threads that concurrent Sparkplug can use (0 for unbounded)")
 #else
 DEFINE_BOOL(baseline_batch_compilation, false, "batch compile Sparkplug code")