Remove unused Gradle antlr plugin

B2Program doesn't need to use ANTLR directly. antlr-parser already
contains the generated parser and has its own dependency on ANTLR.

build.gradle

@@ -2,7 +2,6 @@ import org.gradle.api.tasks.testing.logging.TestLogEvent
 
 apply plugin: 'java'
 apply plugin: 'application'
-apply plugin: 'antlr'
 apply plugin: 'eclipse'
 apply plugin: 'idea'