[build] Remove constexpr mutex define on Windows

[Gold856]

Since we ship a newer runtime, and we also have checks to ensure a valid runtime, we can remove this for the last time.

[Gold856]

I used the artifacts from this PR and loaded it in a robot project. I have this in the constructor:

  public Robot() {
    System.out.println(WPILibVersion.Version);
    new TalonFX(0);
  }

I get our custom error when VS Code isn't configured to point to our JDK:

When I add

  "java.configuration.runtimes": [
    {
      "name": "JavaSE-17",
      "path": "C:\\Users\\Public\\wpilib\\2025\\jdk"
    }
  ]

to my VS Code config, my robot project starts up normally.

The alpha tag is because my fork doesn't have all the tags the main repo does.
WPILib Information:
Project Version: 2025.1.1-beta-1
VS Code Version: 1.94.2
WPILib Extension Version: 2025.1.1-beta-1
C++ Extension Version: 1.22.9
Java Extension Version: 1.36.2024092708
Java Debug Extension Version: 0.58.2024090204
Java Dependencies Extension Version 0.24.0
Java Version: 17
Java Location: C:\Users\Public\wpilib\2025\jdk
Vendor Libraries:
CTRE-Phoenix (v6) (25.0.0-beta-2)
photonlib (v2025.0.0-beta-1)
REVLib (2025.0.0-beta-1)
WPILib-New-Commands (1.0.0)