cxl/pci: Remove outdated FIXME comment and BUILD_BUG_ON

cxl/pci: Remove outdated FIXME comment and BUILD_BUG_ON

Remove the outdated FIXME comment about switching to struct_group() and
the associated BUILD_BUG_ON check. This work was already completed in
commit 301e68dd9b9b ("cxl/core: Replace unions with struct_group()")
which converted struct cxl_regs to use struct_group_tagged().

The BUILD_BUG_ON was checking that anonymous union layout was preserved,
but since struct_group() now handles this correctly, the compile-time
check is no longer necessary.

Signed-off-by: Samasth Norway Ananda <samasth.norway.ananda@oracle.com>
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20260105203833.1604585-1-samasth.norway.ananda@oracle.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>