VS: Revert "Build custom commands concurrently when possible"

Since commit 33c15ae2b9 (VS: Build custom commands concurrently when
possible, 2023-01-19, v3.26.0-rc1~56^2) we add `BuildInParallel` to
custom commands in `.vcxproj` files.  However, this can break existing
projects that implicitly rely on serial execution of custom commands.
For example, custom commands in our FindCUDA module run MSVC (via nvcc)
with a common `vc*.pdb` file, and therefore cannot run in parallel.

Revert use of `BuildInParallel` while leaving most of the infrastructure
for it in place.  It can be restored later with an option or policy.

Fixes: #24576
Issue: #18405

Source/cmVisualStudio10TargetGenerator.cxx

@@ -1806,13 +1806,10 @@ void cmVisualStudio10TargetGenerator::WriteCustomRule(
       this->WriteCustomRuleCSharp(e0, c, name, script, additional_inputs.str(),
                                   outputs.str(), comment, ccg);
     } else {
-      this->WriteCustomRuleCpp(
-        *spe2, c, script, additional_inputs.str(), outputs.str(), comment, ccg,
-        symbolic,
-        (command.GetUsesTerminal() ||
-         (command.HasMainDependency() && source->GetIsGenerated()))
-          ? BuildInParallel::No
-          : BuildInParallel::Yes);
+      // FIXME(#18405): Enable BuildInParallel::Yes via an option or policy.
+      this->WriteCustomRuleCpp(*spe2, c, script, additional_inputs.str(),
+                               outputs.str(), comment, ccg, symbolic,
+                               BuildInParallel::No);
     }
   }
 }