libvulkan: remove a redundant check debuggable logic

There're below cases which we can load the debug layers:
(1) Layer search paths added from Settings global
    Layers enabled from Settings global
(2) Layer search paths added from default system debug path
    Layers enabled from system property
(3) Layer search paths are from app class loader's default paths
    Layers enabled from system property

For (1), we have done the debuggable check in Java land GraphicsEnvironment.

For (2), we have a separate check debuggable logic for adding the system
debug path to the layers search paths.

For (3), we should never need to check debuggable.

So we should just remove the redundant debuggable check when we add implicit
layers, which can unblock (3) immediately as well as maintaining the security
logic around debug layers.

Bug: 147838267
Bug: 149183377
Test: CtsRootlessGpuDebugHostTest#testSystemPropertyEnableVulkan on userbuild
Change-Id: I80d9cf3ff6786ce4d4ec5d8afb8da58ba0842ec9