Remove javassist dependency, close AsyncHttpClient#1362

Motivation:

Netty’s removing it, as perf improvement was never proven.

Modification:

Remove dep from pom

Result:

Smaller dependency tree

Author: slandelle

client/pom.xml

@@ -53,10 +53,5 @@
 			<artifactId>netty-reactive-streams</artifactId>
 			<version>2.0.0-M1</version>
 		</dependency>
-		<dependency>
-			<groupId>org.javassist</groupId>
-			<artifactId>javassist</artifactId>
-			<version>3.21.0-GA</version>
-		</dependency>
 	</dependencies>
 </project>