meteor3: remove dead chimp e2e runner (pulled fibers, broke Docker build)

chimp 0.51.1 (Selenium 2 / cucumber e2e runner) was the last dependency
pulling native fibers@1.x, which cannot compile on Node 22 and failed the
clean Docker image build (node-gyp: no prebuilt, no python). Not imported
anywhere. Also add python-is-python3 to the builder stage so node-gyp can
find python for any other native dep. Boot + REST smoke green.

package.json

@@ -12,7 +12,6 @@
     "babel-runtime": "^6.26.0",
     "bcrypt": "^5.1.1",
     "bootstrap-carousel-swipe": "0.0.6",
-    "chimp": "^0.51.1",
     "commonmark": "^0.28.1",
     "core-js": "^2.5.1",
     "crypto-random-hex": "^1.0.0",