nvk: remove unused relative_dir variable

In commit f35172b6a4 ("meson: make CL args common"), the
fmacro-prefix-map using relative_dir variable was moved from child
meson.build files to the main meson.build file. However, the
relative_dir variable assignment from src/nouveau/vulkan/meson.build
wasn't removed, while src/asahi/libagx's and src/panfrost/libpan's were.
This seems to be an oversight, so let's remove this seemingly now unused
relative_dir meson variable from nvk's meson.build.

This also allows us to remove the use of discouraged global_build_root()
[1] and global_source_root()[2].

[1] https://mesonbuild.com/Reference-manual_builtin_meson.html#mesonglobal_build_root
[2] https://mesonbuild.com/Reference-manual_builtin_meson.html#mesonglobal_source_root

Fixes: f35172b6a4 ("meson: make CL args common")
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36927>
(cherry picked from commit e1765a2914)

.pick_status.json

@@ -4184,7 +4184,7 @@
         "description": "nvk: remove unused relative_dir variable",
         "nominated": true,
         "nomination_type": 2,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "f35172b6a46558fc628dcc18d41f1a036f93fa1f",
         "notes": null

src/nouveau/vulkan/meson.build

@@ -93,8 +93,6 @@ nvk_entrypoints = custom_target(
   depend_files : vk_entrypoints_gen_depend_files,
 )
 
-relative_dir = fs.relative_to(meson.global_source_root(), meson.global_build_root())
-
 nvkcl_spv = custom_target(
   input : nvkcl_files,
   output : 'nvkcl.spv',