Remove unused `watch:test:unit` script (#8960)

This script used `nodemon`, which is not among our dependencies, so it
doesn't work.

package.json

@@ -15,7 +15,6 @@
     "dapp-chain": "GANACHE_ARGS='-b 2' concurrently -k -n ganache,dapp -p '[{time}][{name}]' 'yarn ganache:start' 'sleep 5 && yarn dapp'",
     "forwarder": "node ./development/static-server.js ./node_modules/@metamask/forwarder/dist/ --port 9010",
     "dapp-forwarder": "concurrently -k -n forwarder,dapp -p '[{time}][{name}]' 'yarn forwarder' 'yarn dapp'",
-    "watch:test:unit": "nodemon --exec \"yarn test:unit\" ./test ./app ./ui",
     "sendwithprivatedapp": "node development/static-server.js test/e2e/send-eth-with-private-key-test --port 8080",
     "test:unit": "mocha --exit --require test/env.js --require test/setup.js --recursive \"test/unit/**/*.js\" \"ui/app/**/*.test.js\"",
     "test:unit:global": "mocha --exit --require test/env.js --require test/setup.js --recursive mocha test/unit-global/*",