For #4132: remove raptor property and isRaptorEnabled manifest placeholder.

afaict, this is only used for BrowserPerformanceTestActivity, which is
being removed in this PR.

app/build.gradle

@@ -35,7 +35,6 @@ android {
         def deepLinkSchemeValue = "fenix-dev"
         buildConfigField "String", "DEEP_LINK_SCHEME", "\"$deepLinkSchemeValue\""
         manifestPlaceholders = [
-                "isRaptorEnabled": "false",
                 "deepLinkScheme": deepLinkSchemeValue
         ]
     }
@@ -56,12 +55,10 @@ android {
             shrinkResources false
             minifyEnabled false
             applicationIdSuffix ".fenix.debug"
-            manifestPlaceholders.isRaptorEnabled = "true"
             resValue "bool", "IS_DEBUG", "true"
             pseudoLocalesEnabled true
         }
         forPerformanceTest releaseTemplate >> { // the ">>" concatenates the raptor-specific options with the template
-            manifestPlaceholders.isRaptorEnabled = "true"
             applicationIdSuffix ".fenix.performancetest"
             debuggable true
             manifestPlaceholders = [
@@ -556,10 +553,6 @@ dependencies {
     lintChecks project(":mozilla-lint-rules")
 }
 
-if (project.hasProperty("raptor")) {
-    android.defaultConfig.manifestPlaceholders.isRaptorEnabled = "true"
-}
-
 if (project.hasProperty("coverage")) {
     tasks.withType(Test) {
         jacoco.includeNoLocationClasses = true