test_vulkan: fix build flags and warnings (#2443)

- Do not override parent-provided value for CMAKE_CXX_FLAGS
- Stop building with -fpermissive (not required/bad practice)
- Delete unused variables
- Remove unnecessary casts

Signed-off-by: Kévin Petit <kpet@free.fr>

test_conformance/vulkan/CMakeLists.txt

@@ -2,7 +2,6 @@ set (MODULE_NAME VULKAN)
 
 list(APPEND CLConform_LIBRARIES vulkan_wrapper)
 set(CMAKE_COMPILE_WARNING_AS_ERROR OFF)
-set(CMAKE_CXX_FLAGS "-fpermissive")
 if(WIN32)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DVK_USE_PLATFORM_WIN32_KHR")
 endif(WIN32)