Fix remaining dead code and stale comments in stackwalker refactor

[jbachorik]

What does this PR do?:
Fixes three leftover issues from the stackwalker refactor (#451):

• Remove Profiler::_runtime_stubs (CodeCache): the field was missed when _stubs_lock, _call_stub_begin, and _call_stub_end were removed. All stub routing moved to JitCodeCache in Refactor stackwalker #451; profiler.cpp has zero remaining references to this field.
• Update the alignas comment on Profiler that still listed _stubs_lock after its removal.
• Fix grammar in HotspotSupport::checkFault: "have not been initialized" → "has not been initialized".

Motivation:
Follow-up from code review of #451. These were identified in the review but not addressed in the main PR's code-review commit.

Additional Notes:
Depends on #451.

How to test the change?:
No behaviour change — dead field removal and comment fixes only. Existing tests cover the surrounding code.

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.
• JIRA: [JIRA-XXXX]

Unsure? Have a question? Request a review!

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🤖 Generated with Claude Sonnet on behalf of @jbachorik

[dd-octo-sts]

Scan-Build Report

User:	runner@runnervm727z3
Working Directory:	/home/runner/work/java-profiler/java-profiler/ddprof-lib/src/test/make
Command Line:	make -j4 all
Clang Version:	Ubuntu clang version 18.1.3 (1ubuntu1)
Date:	Thu Apr 9 09:13:45 2026

Bug Summary

Bug Type	Quantity	Display?
All Bugs	1
Logic error
Stack address stored into global variable	1

Reports

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
Logic error	Stack address stored into global variable	hotspotSupport.cpp	walkVM	701	37

[dd-octo-sts]

CI Test Results

Run: #24182253808 | Commit: ea62ea7 | Duration: 22m 22s (longest job)

✅ All 32 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 32 | Passed: 32 | Failed: 0

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Updated: 2026-04-09 10:15:40 UTC