Remove `yarn` dependency

[augnustin]

Trying to install @graphprotocol/indexer-agent, I get the following error:

npm error code 127
npm error path /home/augustin/.npm-global/lib/node_modules/@graphprotocol/indexer-cli/node_modules/@graphprotocol/cost-model
npm error command failed
npm error command sh -c node-pre-gyp install --fallback-to-build=false --update-binary || yarn build-release
npm error response status 404 Not Found on https://github.com/graphprotocol/agora/releases/download/v0.1.18/graphprotocol-agora-plugin-v0.1.18-node-v127-linux-x64.tar.gz
npm error node-pre-gyp info it worked if it ends with ok
npm error node-pre-gyp info using node-pre-gyp@1.0.11
npm error node-pre-gyp info using node@22.13.1 | linux | x64
npm error (node:3493681) [DEP0040] DeprecationWarning: The `punycode` module is deprecated. Please use a userland alternative instead.
npm error (Use `node --trace-deprecation ...` to show where the warning was created)
npm error node-pre-gyp http GET https://github.com/graphprotocol/agora/releases/download/v0.1.18/graphprotocol-agora-plugin-v0.1.18-node-v127-linux-x64.tar.gz
npm error node-pre-gyp ERR! install response status 404 Not Found on https://github.com/graphprotocol/agora/releases/download/v0.1.18/graphprotocol-agora-plugin-v0.1.18-node-v127-linux-x64.tar.gz 
npm error node-pre-gyp ERR! install error 
npm error node-pre-gyp ERR! stack Error: response status 404 Not Found on https://github.com/graphprotocol/agora/releases/download/v0.1.18/graphprotocol-agora-plugin-v0.1.18-node-v127-linux-x64.tar.gz
npm error node-pre-gyp ERR! stack     at /home/augustin/.npm-global/lib/node_modules/@graphprotocol/indexer-cli/node_modules/@mapbox/node-pre-gyp/lib/install.js:67:15
npm error node-pre-gyp ERR! stack     at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
npm error node-pre-gyp ERR! System Linux 6.8.0-52-generic
npm error node-pre-gyp ERR! command "/usr/bin/node" "/home/augustin/.npm-global/lib/node_modules/@graphprotocol/indexer-cli/node_modules/.bin/node-pre-gyp" "install" "--fallback-to-build=false" "--update-binary"
npm error node-pre-gyp ERR! cwd /home/augustin/.npm-global/lib/node_modules/@graphprotocol/indexer-cli/node_modules/@graphprotocol/cost-model
npm error node-pre-gyp ERR! node -v v22.13.1
npm error node-pre-gyp ERR! node-pre-gyp -v v1.0.11
npm error node-pre-gyp ERR! not ok 
npm error sh: 1: yarn: not found

I'm not clear why yarn would have to be a technical dependency, while npm does the job. Especially for global packages installations.

Cheers