debian,cmake: Add libxsimd-dev and enable system xsimd for Arrow

debian,cmake: Add libxsimd-dev and enable system xsimd for Arrow

Arrow requires xsimd >= 9.0.1 according to arrow/cpp/thirdparty/versions.txt.
This commit makes two related changes:

1. Add libxsimd-dev as a build dependency in debian/control when
   pkg.ceph.arrow is not selected. This satisfies the vendored Arrow build
   requirements and enables successful builds in network-restricted
   environments.

2. Remove the hardcoded -Dxsimd_SOURCE=BUNDLED from BuildArrow.cmake.
   Since we already set ARROW_DEPENDENCY_SOURCE=SYSTEM, Arrow will
   automatically try to find the system xsimd package and fall back to
   the bundled version if not found or insufficient.

This allows systems with libxsimd-dev >= 9.0.1 to use the system package,
reducing build time and dependencies, while maintaining compatibility with
older distributions that will automatically use the bundled version.

Distribution availability:
- Ubuntu Noble (24.04): 12.1.1 (sufficient)
- Ubuntu Jammy (22.04): 7.6.0 (insufficient, will use bundled)
- Debian Trixie (13): 13.2.0 (sufficient)

Signed-off-by: Kefu Chai <k.chai@proxmox.com>