[core-gl-kit] media-libs/mesa: remove obsolete XA USE flag, no longer supported in Mesa 26.x

Mesa 26.x no longer provides XA support, but the ebuild still exposes IUSE=xa, which is now obsolete and misleading.

Proof from the build of Mesa 26.0.0:

```
Gallium
Drivers : crocus i915 iris llvmpipe nouveau r300 r600 radeonsi svga
Frontends : mesa
```

There is no XA frontend listed. If XA were present, it would appear as xa in the Gallium frontends.

Additionally, the Meson configuration shows no XA-related option:

```
User defined options:
gallium-drivers : crocus,i915,iris,llvmpipe,nouveau,r300,r600,radeonsi,svga
```

No gallium-xa or equivalent flag is available or enabled.

At runtime, this results in:

```
pkg-config --modversion xatracker
Package 'xatracker' not found
```

This confirms that:

* libxatracker.so is not built
* xatracker.pc is not installed
* XA has been removed upstream

Keeping IUSE=xa in the ebuild suggests functionality that no longer exists and leads to broken builds or misconfiguration in consumers such as xf86-video-vmware.

The xa USE flag should be removed from the ebuild to reflect the current upstream state of Mesa.

More infos: XA is no longer built by default in Mesa 25.1 and will be removed completely in Mesa 25.2 - https://gitlab.archlinux.org/archlinux/packaging/packages/xf86-video-vmware/-/work_items/3

Find the media-libs/mesa ebuild and inspect its IUSE definition for the obsolete xa flag. Remove that flag, then verify the package no longer exposes XA-related configuration and that the ebuild reflects the Mesa 26.x build output described in the issue.

Written by the indexing model from the issue text.