chore: Stop constructing FileCoverage in noop instrumenter.

This makes the behavior consistent with the real instrumenter which just
provides raw data.

lib/instrumenters/noop.js

@@ -1,12 +1,11 @@
-const { FileCoverage } = require('istanbul-lib-coverage').classes
 const { readInitialCoverage } = require('istanbul-lib-instrument')
 
 function NOOP () {
   return {
     instrumentSync (code, filename) {
       const extracted = readInitialCoverage(code)
       if (extracted) {
-        this.fileCoverage = new FileCoverage(extracted.coverageData)
+        this.fileCoverage = extracted.coverageData
       } else {
         this.fileCoverage = null
       }