Remove unnecessary source sets

These source sets are not configured correctly (no classpaths for example) and
they subsequently cannot compile. The special tests are put into separate tasks that
run from the same source set (test).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=222080273

gradle/core/build.gradle

@@ -80,24 +80,6 @@ sourceSets {
       exclude testResourceExclude
     }
   }
-  fragileTest {
-    java {
-      srcDirs testSourceDirs
-    }
-    resources {
-      srcDirs testResourceDirs
-      exclude testResourceExclude
-    }
-  }
-  outcastTest {
-    java {
-      srcDirs testSourceDirs
-    }
-    resources {
-      srcDirs testResourceDirs
-      exclude testResourceExclude
-    }
-  }
 }
 
 configurations {