target-i386: drop usage of prev_debug_excp_handler

Chains of exception handlers are currently unused feature, drop it
for now so as not to expose prev_debug_excp_handler at global
scope when moving tcg initialization into target-i386/cpu.c

Later we probably could re-invent better interface for this.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
