Remove span.h from meson install_headers

The header was deleted but not removed from the meson build
configuration, causing the C++ Meson CI to fail.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 2 people

cpp/src/arrow/util/meson.build

@@ -159,7 +159,6 @@ install_headers(
         'rows_to_batches.h',
         'simd.h',
         'small_vector.h',
-        'span.h',
         'string_util.h',
         'string.h',
         'task_group.h',