Remove unused legacy three.js viewer (viewer/legacy)

Summary:
Deletes `fbcode/mapillary/opensfm/viewer/legacy/`: a standalone `reconstruction.html` plus vendored copies of `three.js` (18.5k lines), `OrbitControls.js`, `TrackballControls.js`, `dat.gui.js` and `jquery.js`.

This viewer was replaced by the MapillaryJS-based viewer (`viewer/server.py` + `viewer/src`) in 2021. Nothing references `legacy/` any more: no BUCK target, docs, `server.py`, or other code (CodeHub grep for `viewer/legacy` under `fbcode/mapillary`: no hits). `doc/source/using.rst` only documents `viewer/server.py`.

These vendored files account for 6 of the mapillary project's RLAIR P0 issues: `large_file` for `three.js`, and `high_cyclomatic_complexity_max_function` for `three.js`, `dat.gui.js`, `OrbitControls.js` and `TrackballControls.js`. Refactoring third-party code is not an option.

OpenSfM is open source. If the GitHub mirror should keep shipping the legacy viewer, abandon this diff and ask for these paths to be excluded from RLAIR instead.

Reviewed By: millenbop

Differential Revision: D125236453

fbshipit-source-id: 010d318f6e77ff5f798fc9d00f4b4f5b5c0607c8

Author: caglarpir