vendor.xilinx_{7series,ultrascale}: remove `grade` property.

vendor.xilinx_{7series,ultrascale}: remove `grade` property.

This was added in commit bfd4538d based on a misunderstanding of how
Xilinx part numbers work.
 * non-ultrascale 7-series parts don't have temperature grades;
 * ultrascale parts have temperature grade as a part of speed grade.