fix: remove postinstall script

[EastSun5566]

postinstall script will cause npm i -g to fail:

npm i -g @hackmd/spm
npm error code 1
npm error path ~/.nvm/versions/node/v24.16.0/lib/node_modules/@hackmd/spm
npm error command failed
npm error command sh -c bun run build
npm error $ bun build index.ts --target node --outdir dist
npm error error: ModuleNotFound resolving "index.ts" (entry point)
npm error error: script "build" exited with code 1

When global install triggers the build script, it causes the install to fail. I think the build can be run manually.