[Doc] java.awt.im.InputContext API doc has a reference to extensions

Summary

Remove reference to the extension mechanism in java.awt.im.InputContext javadoc.

Problem

java/awt/im/InputContext.java still mentions about loading input methods as extensions. With the "JEP 220 Modular Run-Time Images", the extension mechanism was removed, and Input methods are now loaded from application class path.

Solution

Solution is to update API Doc of java.awt.im.InputContext.java and remove reference to "extensions" .

Specification

Changing API spec of java/awt/im/InputContext.java

from :

 *
 * <p>
 * The Java Platform supports input methods that have been developed in the Java
 * programming language, using the interfaces in the {@link java.awt.im.spi} package,
 * and installed into a Java SE Runtime Environment as extensions. 

to :

*
* <p>
* The Java Platform supports input methods that have been developed in the Java
* programming language, using the interfaces in the {@link java.awt.im.spi} package,
* which can be made available by adding them to the application's class path.