Remove `bimg` thumbnailer #262

Job	Run time
Linting	2m 9s
Unit tests	1m 56s
Build for Windows (windows, amd64)	4m 41s
Build for Linux (linux, amd64)	39s
Build for Linux (linux, 386)	1m 3s
WASM build test	0s
Initial tests passed	1s
Upgrade tests	7m 8s
Sytest (SQLite native)	5m 20s
Sytest (SQLite Cgo)	5m 10s
Integration tests	4m 29s
Sytest (PostgreSQL)	6m 44s
Complement (SQLite native)	4m 23s
Upgrade tests from HEAD-2	7m 12s
Complement (SQLite Cgo)	4m 50s
Complement (PostgreSQL)	8m 28s
Integration tests passed	2s
Update Docker images	0s
	1h 4m 15s

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