Fix-dsig tool missing. Update docs

[kateliev]

@tphinney while doing build today this one popped up as a new kind of problem. It seems that G is no longer supporting the tool and the operation step is unneeded (?). See here for reference: https://github.com/googlefonts/gftools/issues/564 so the tool was missing in the latest gftools.

It is up for you to decide if that step should updated/removed from our build documentation. For now i am going to exclude that step from our build scripts and will add the table as dropped to the sg-fix-unwanted-tables.py tool.

In case you decide to keep the step, here is a hard-copy of the tool that we should add to our scripts folder: https://github.com/googlefonts/gftools/blob/v0.8.8/bin/gftools-fix-dsig.py