Use two digit numbers for SignifyDelete signs

Defining the SignifyDelete set of signs on the fly provides the ability
to specify the exact line deletion count for up to 99 lines, rather than
the current limit of 9 lines.

This also has the benefit of reducing the number of signs defined when
there aren't many hunks of deleted lines.  On the flip side, if there
are other sign-placing plugins in use, then it also increases the chance
of nearing the ~120 sign definition limit if there are many hunks of
varying line deletions.