Revert "fix(server): application/json header for notifictions" by mattzcarey · Pull Request #1388

Revert "fix(server): application/json header for notifictions" by mattzcarey · Pull Request #1388 · modelcontextprotocol/typescript-sdk

Resolved conflicts:
- packages/server/src/server/webStandardStreamableHttp.ts: deleted in main
- packages/server/test/server/streamableHttp.test.ts: took main's version

Applied revert change to packages/server/src/server/streamableHttp.ts:
- Removed Content-Type: application/json header from 202 notification responses