[unwanted_tables] Should not check for DSIG table

[khaledhosny]

Observed behaviour

The check will fail if the font has a DSIG table.

Expected behaviour

All other tables in the checks (excluding prop, see #4989) are debugging or tool-specific tables that should not be in production fonts, while the rationale for DSIG is different and there may be legitimate cases for wanting to include a DSIG table in a font. By including tables with different rationales in the same check, projects that need DSIG are forced to disable the check and risk missing the inclusion of other unwanted tables.

[khaledhosny]

I wanted so say, DSIG can be in a check of its own, just not included with the other tables in the same check.