Remove `DELETE_DEREF` bytecode instruction

[markshannon]

Feature or enhancement

Proposal:

The DELETE_DEREF opcode can be removed and deletion implemented by the sequence:
LOAD_DEREF POP_TOP PUSH_NULL STORE_DEREF

This would free up an opcode, and, since deletion of non-locals variables is practically never used, there is no cost to doing this.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere