(petitioner, stockbroker, could not exclude discharge of indebtedness income attributable to interest on loan forgiven by employer; I.R.C. Sec. 108(e)(2) only excludes from income interest that would have given rise to a deduction; petitioner used loan for income production purposes, but failed to prove deductibility under I.R.C. Sec. 212 as an expense associated with income production; in any event, interest deduction would have been limited by I.R.C. Sec. 163(d)).