[wallet] Add ProcessImportLegacy()

Summary:
This commit adds a ProcessImportLegacy() function which
currently does nothing. It also unindents a block of
code for a future move-only change.

Reviewer hint: review with -w to ignore whitespace changes.

This is a partial backport of Core PR14491 : https://github.com/bitcoin/bitcoin/pull/14491/commits/4cac0ddd258bc82258ccc99568d02d3b2415339d

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6189