Remove unused dependencies (#180)

Author: bakkot

package-lock.json

@@ -35,7 +35,6 @@
 		"chalk": "^1.1.3",
 		"ecmarkdown": "^3.0.9",
 		"grammarkdown": "^2.0.12",
-		"he": "^1.2.0",
 		"highlight.js": "^9.17.1",
 		"html-escape": "^1.0.2",
 		"js-yaml": "^3.13.1",
@@ -47,7 +46,6 @@
 	"devDependencies": {
 		"@types/bluebird": "^3.5.29",
 		"@types/chalk": "^0.4.31",
-		"@types/he": "^1.1.0",
 		"@types/highlight.js": "^9.12.3",
 		"@types/js-yaml": "^3.12.1",
 		"@types/jsdom": "^11.12.0",
@@ -56,7 +54,6 @@
 		"@typescript-eslint/eslint-plugin": "^2.21.0",
 		"@typescript-eslint/parser": "^2.21.0",
 		"auto-changelog": "^1.16.2",
-		"diff": "^2.2.3",
 		"eslint": "^6.8.0",
 		"mocha": "^5.2.0",
 		"safe-publish-latest": "^1.1.4",