Plugin: remove superfluous cancelable attribute

This attribute was intended for use with ImageJ commands, but there is
already a dedicated interface called Cancelable for signaling whether
the command can be canceled. So the attribute is unnecessary.

Author: ctrueden

src/main/java/org/scijava/plugin/Plugin.java

@@ -147,9 +147,6 @@
 	/** When false, the plugin is not displayed in the user interface. */
 	boolean visible() default true;
 
-	/** When false, the user interface will not provide a cancel button. */
-	boolean cancelable() default true;
-
 	/**
 	 * Provides a "hint" as to whether the plugin would behave correctly in a
 	 * headless context.