[D3D12] Switch from gflags to cvars

src/xenia/app/premake5.lua

@@ -12,7 +12,6 @@ project("xenia-app")
     "capstone",
     "dxbc",
     "discord-rpc",
-    "gflags",
     "glew",
     "glslang-spirv",
     "imgui",
@@ -46,9 +45,6 @@ project("xenia-app")
     "XBYAK_NO_OP_NAMES",
     "XBYAK_ENABLE_OMITTED_OPERAND",
   })
-  includedirs({
-    project_root.."/third_party/gflags/src",
-  })
   local_platform_files()
   files({
     "xenia_main.cc",
@@ -92,6 +88,5 @@ project("xenia-app")
     if not os.isfile(user_file) then
       debugdir(project_root)
       debugargs({
-        "--flagfile=scratch/flags.txt",
       })
     end