[spec] Remove FIRRTL Specification (#2488)

This removes the FIRRTL specification as this now lives in its own
repository:

    https://github.com/chipsalliance/firrtl-spec



All git history (excluding binary updates to spec.pdf) have been
migrated to the new repository.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>