Remove no-op TIME_INIT on deser

Summary:
Assigning TIME_INIT to nTime was needed to fully re-initialize a dirty
object where the deserialization might skip nTime.
See https://github.com/bitcoin/bitcoin/pull/19020/files#r427620111

Now that the without-nTime logic is removed in commit
dbcb5742c48fd26f77e500291d7083e12eec741b and commit
e08770bed187bfa66f525d42e484579bcea78bba, the logic here can be removed
as well.

Also, remove confusing and redundant preprocessor code.

This is a backport of core#25245

Depends on D15090

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15091