Remove inotify-based TLS priming thread directory watcher

[jbachorik]

What does this PR do?:
Removes the inotify-based thread directory watcher from the TLS priming system. This includes removing the startThreadDirectoryWatcher() and stopThreadDirectoryWatcher() functions along with all associated implementation code across Linux and macOS platforms.

Motivation:
The filesystem-based thread monitoring using inotify was causing performance overhead in production environments. This functionality will be replaced with library patching in the near future, providing a more efficient approach to native thread TLS initialization.

Additional Notes:

• The TLS priming infrastructure for Java threads via JVMTI remains intact and fully functional
• Native threads will now use lazy initialization until lib patching is implemented
• All signal handler infrastructure remains available for future lib patching use
• Documentation has been updated to reflect these changes and document the removal rationale

Changes made:

• Removed watcher thread loop and inotify monitoring from os_linux_dd.cpp
• Removed watcher stubs from os_macos_dd.cpp
• Removed watcher function declarations from os_dd.h
• Updated thread.cpp to remove watcher initialization and cleanup calls
• Removed ThreadDirectoryWatcher test from test_tlsPriming.cpp
• Updated TlsPriming.md documentation to reflect the removal and document future direction

How to test the change?:
All C++ unit tests pass successfully, including the TLS priming tests:

• Built and ran gtestDebug - all 93 C++ unit tests passed
• TLS priming tests verify signal handler installation, thread enumeration, and signal delivery
• JVMTI-based thread initialization remains tested and functional

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.

Unsure? Have a question? Request a review!

[zhengyu123]

LGTM