Revert "sysutils/cpu-microcode-intel: Handle extended signature tables"

This reverts a change to the way Intel CPU microcode files were split.
In d4b93e8846, an explicitly named file was written for each
signature+platform combination in an image's extended signature table.
Writing these new files offered discoverability advantages, but it was
unnecessary since cpucontrol(8) already checks the embedded extended
signature table. Moreover, adding these extra files increased the size
of the split files by about 50% when the default SPLIT knob was enabled.

This reverts commit d4b93e88468975e64d6321cfc463f5fc19f46d35.

PR: 295351
Sponsored by: The FreeBSD Foundation