Remove 'flow-coverage-report' script. (facebook#13395)

`flow-coverage-report` stopped working after Flow was set to run for each renderer separately (facebook#12846). As discussed in facebook#13393, this is hard to fix without adding complexity to `.flowconfig`'s generation.

Author: elas7

.flowcoverage

@@ -9,7 +9,6 @@ __benchmarks__
 build/
 remote-repo/
 coverage/
-flow-coverage/
 .module-cache
 fixtures/dom/public/react-dom.js
 fixtures/dom/public/react.js

.gitignore

@@ -55,7 +55,6 @@
     "fbjs-scripts": "^0.8.3",
     "filesize": "^3.5.6",
     "flow-bin": "^0.72.0",
-    "flow-coverage-report": "^0.4.0",
     "glob": "^6.0.4",
     "glob-stream": "^6.1.0",
     "google-closure-compiler": "20180506.0.0",
@@ -94,7 +93,6 @@
   },
   "scripts": {
     "build": "npm run version-check && node ./scripts/rollup/build.js",
-    "flow-coverage": "flow-coverage-report --config ./.flowcoverage",
     "linc": "node ./scripts/tasks/linc.js",
     "lint": "node ./scripts/tasks/eslint.js",
     "lint-build": "node ./scripts/rollup/validate/index.js",