Revert "Implement gRFC A97: xDS JWT Call Credentials (grpc#12951)" (grpc#13017)

This reverts commit 1053f4b. We need to
address the concerns raised in grpc#13006.

Author: kannanjgithub

auth/BUILD.bazel

@@ -11,7 +11,6 @@ java_library(
         "//api",
         artifact("com.google.auth:google-auth-library-credentials"),
         artifact("com.google.code.findbugs:jsr305"),
-        artifact("com.google.code.gson:gson"),
         artifact("com.google.guava:guava"),
     ],
 )

auth/build.gradle

@@ -17,8 +17,7 @@ tasks.named("jar").configure {
 dependencies {
     api project(':grpc-api'),
             libraries.google.auth.credentials
-    implementation libraries.guava,
-            libraries.gson
+    implementation libraries.guava
     testImplementation project(':grpc-testing'),
             project(':grpc-core'),
             project(":grpc-context"), // Override google-auth dependency with our newer version