[libunwind] Remove unused _LIBUNWIND_HAS_NO_THREADS macro in tests

The _LIBUNWIND_HAS_NO_THREADS macro is only picked up by libunwind
inside its sources, so it is only required when it builds. It doesn't
need to be defined when running the tests.

Author: ldionne

libunwind/test/configs/llvm-libunwind-merged.cfg.in

@@ -25,7 +25,6 @@ if @LIBUNWIND_USES_ARM_EHABI@:
     config.available_features.add('libunwind-arm-ehabi')
 
 if not @LIBUNWIND_ENABLE_THREADS@:
-    compile_flags.append('-D_LIBUNWIND_HAS_NO_THREADS')
     config.available_features.add('libunwind-no-threads')
 
 if @LIBUNWIND_ENABLE_CET@:

libunwind/test/configs/llvm-libunwind-shared.cfg.in

@@ -28,7 +28,6 @@ if @LIBUNWIND_USES_ARM_EHABI@:
     config.available_features.add('libunwind-arm-ehabi')
 
 if not @LIBUNWIND_ENABLE_THREADS@:
-    compile_flags.append('-D_LIBUNWIND_HAS_NO_THREADS')
     config.available_features.add('libunwind-no-threads')
 
 if @LIBUNWIND_ENABLE_CET@:

libunwind/test/configs/llvm-libunwind-static.cfg.in

@@ -29,7 +29,6 @@ if @LIBUNWIND_USES_ARM_EHABI@:
     config.available_features.add('libunwind-arm-ehabi')
 
 if not @LIBUNWIND_ENABLE_THREADS@:
-    compile_flags.append('-D_LIBUNWIND_HAS_NO_THREADS')
     config.available_features.add('libunwind-no-threads')
 else:
     link_flags.append('-lpthread')