db_nextframe/amd64: remove TRAP_INTERRUPT frame type

Besides the confusing name, this type is effectively unused.
In all cases where it could be set, the INTERRUPT type is set by the
earlier code. The conditions for TRAP_INTERRUPT are a subset of the
conditions for INTERRUPT.

Reviewed by: kib, markj
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D22305