Files app: remove unused LoaderType enum (CANVAS)

Nominally, there are two enum values: IMAGE and CANVAS. IMAGE is the
default (if passing undefined to the ThumbnailLoader constructor). The
only use of CANVAS is in test code (thumbnail_loader_unittest.ts) and
the tests are still green when passing IMAGE instead.

"CANVAS is only used in tests" is true in 2024 (today) and also true in
2018 (e.g. grepping the ui/file_manager/file_manager source code as of
commit f919e4565e1f3960548400a1349a23ee75ff7866,
https://chromium-review.googlesource.com/1212706).

That commit (in 2018) authored the thumbnail_loader_unittest.ts line 6
that is today (in 2024) the only place that passes CANVAS. But its use
of CANVAS (at line 6) is possibly copy/pasting itself (from line 204,
see link below). I suspect that the test doesn't actually care about the
IMAGE vs CANVAS choice.

https://chromium-review.googlesource.com/c/chromium/src/+/1212706/6/ui/file_manager/file_manager/foreground/js/thumbnail_loader_unittest.js

Bug: b/319190834
Change-Id: Ia860b9b5cdbd68cf623f455cfb8ff258b3701486
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5664623
Auto-Submit: Nigel Tao <nigeltao@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1323859}