How can I remove Bootstrap from my Angular project?

First : Remove bootstrap dependency From your package.jsonas shown below for example:

"dependencies": {
    "@angular/animations": "^6.0.0",
    "@angular/common": "^6.0.0",
    "@angular/compiler": "^6.0.0",
    "@angular/core": "^6.0.0",
    "@angular/forms": "^6.0.0",
    "@angular/http": "^6.0.0",
    "@angular/platform-browser": "^6.0.0",
    "@angular/platform-browser-dynamic": "^6.0.0",
    "@angular/router": "^6.0.0",
    "bootstrap": "^3.3.1",   <-------REMOVE THIS
    "core-js": "^2.5.4",
    "rxjs": "6.0.0",
    "zone.js": "^0.8.26"
  },

Second : Remove the bootstrap url from angular.json, if you have placed it under styles section like as shown below

"styles": [
    "src/styles.css",
    "node_modules/bootstrap/dist/css/bootstrap.min.css" <--- REMOVE THIS
],

Third : Check and Remove any bootstrap references explicitly imported in styles.css or any other files remotely or locally like as shown below

@import '~bootstrap/dist/css/bootstrap.min.css';

npm uninstall bootstrap --no-save

also remove the reference you added in angular.json under styles which points bootstrap.min.css

"styles": [
"./node_modules/bootstrap/dist/css/bootstrap.min.css",
"src/styles.css"
 ],