__What__

- Remove `icons_mappings/fa_to_ms.scss` and `icons_mappings/oi_to_ms.scss`,
  along with every `.fa`/`.fa-*` compatibility rule in `icons.scss`.
- Remove `LegacyIconMigrationPlugin` (and its tests), which rewrote
  legacy `fa fa-*` markup into `oi` + `data-icon` at editor startup.

__Why__

The mappings only existed as a transition layer: they let old `fa-*`
markup keep rendering while the codebase was being converted. That
transition is over.

- odoo/odoo#256840 converted every remaining `fa-*` usage to `oi` +
  `data-icon`, so nothing relies on the mapping anymore.
- odoo/odoo#279361 added a CI check preventing any new `fa-*` class from
  being introduced, so the mapping cannot become needed again.

__Note__

`icons.scss` was cleaned up along the way: the now single-purpose mixins
(`icon-content`, `ms-icon`, `oi-icon`, `rotate`, `flip`) were inlined,
duplicated blocks (`.oi-fw`, `.oi-spin`, `.oi-pulse`, the variables
section) were deduplicated, the RTL rules were collapsed into a single
selector list, and the `-webkit-` prefixed transforms/animations were
dropped. The `fa-spin` keyframes are renamed `oi-spin` since the
FontAwesome ones are no longer inherited.

task-5901783