[fenix] For mozilla-mobile/fenix#6931: Change README forPerformanceTe…

…st signingConfig to debug.

fenix/README.md

@@ -105,30 +105,20 @@ You will **need to sign `forPerformanceTest` variants.** For local development,
 
 ```groovy
 android { // this line already exists
-
- signingConfigs {
- forPerformanceTest {
- storeFile file("/Users/<user-name>/.android/debug.keystore")
- storePassword "android"
- keyAlias "androiddebugkey"
- keyPassword "android"
- }
- }
+ // ...
 
  buildTypes { // this line already exists
  // ...
 
  forPerformanceTest releaseTemplate >> { // this line already exists.
  // ...
 
- signingConfig signingConfigs.forPerformanceTest
+ signingConfig signingConfigs.debug
  }
  }
 }
 ```
 
-Where `<user-name>` is your OS user name. This sample assumes you are using macOS: if you're on another OS, please update the path appropriately.
-
 This recommendation will let you use AS just like you do with debug builds but **please do not check in these changes.**
 
 See [perf-frontend-issues#44](https://github.com/mozilla-mobile/perf-frontend-issues/issues/44) for efforts to make performance signing easier.