Merge #13180: Fix re-declared scoped enum as unscoped (Causes issues with some compilers)

Summary:
43f3dec00 Remove enum specifier (to avoid re-declare scoped enum as unscoped) (donaloconnor)

Pull request description:

MSVC fails to compile with the changes made in #10742

The problem is enum types were changed to scoped (`enum class`) but in some places `enum` as an unscoped is used.

This is a very simple fix and I've tested it.

Edit: Had to remove enum altogether - `enum class` doesn't compile on clang.

Backport of Core PR13180

Test Plan:

ninja check

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D7965