Remove bulk operations

[wisepotato]

from #558

Removing the operations experimental feature. We will put it back later but for now it would delay v4 release by too much time. Also the current implementation deviates a lot from the extension that might be included in the v1.1 specs

[maurei]

Will be put back when

• the core of v4 is stable, and
• there is more development resources available
• json:api team finishes working bulk implementation (see JSON:API v1.1 New Directions json-api/json-api#1435)