PHP: remove experimental from Interceptor API (#25788)

diff --git a/src/php/lib/Grpc/CallInvoker.php b/src/php/lib/Grpc/CallInvoker.php
index a1b4553..aacd54e 100644
--- a/src/php/lib/Grpc/CallInvoker.php
+++ b/src/php/lib/Grpc/CallInvoker.php
@@ -21,7 +21,6 @@
 /**
  * CallInvoker is used to pass the self defined channel into the stub,
  * while intercept each RPC with the channel accessible.
- * THIS IS AN EXPERIMENTAL API.
  */
 interface CallInvoker
 {
diff --git a/src/php/lib/Grpc/DefaultCallInvoker.php b/src/php/lib/Grpc/DefaultCallInvoker.php
index e5b1e13..217dafa 100644
--- a/src/php/lib/Grpc/DefaultCallInvoker.php
+++ b/src/php/lib/Grpc/DefaultCallInvoker.php
@@ -20,7 +20,6 @@
 
 /**
  * Default call invoker in the gRPC stub.
- * THIS IS AN EXPERIMENTAL API.
  */
 class DefaultCallInvoker implements CallInvoker
 {
diff --git a/src/php/lib/Grpc/Interceptor.php b/src/php/lib/Grpc/Interceptor.php
index cc465b5..c3bf616 100644
--- a/src/php/lib/Grpc/Interceptor.php
+++ b/src/php/lib/Grpc/Interceptor.php
@@ -23,7 +23,6 @@
  * Represents an interceptor that intercept RPC invocations before call starts.
  * There is one proposal related to the argument $deserialize under the review.
  * The proposal link is https://github.com/grpc/proposal/pull/86.
- * This is an EXPERIMENTAL API.
  */
 class Interceptor
 {