board/xilinx: remove xilinx_2026.1/linux.hash

Now that all Xilinx boards have been bumped to Linux 6.18.40, remove the hash
for the xlnx_rebase_v6.18_LTS_2026.1 release tag.

Signed-off-by: Neal Frager <neal.frager@amd.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>