Do not produce libraries json declarations dump

It's no longer needed for checking binary compatibility.
Clean build of these libraries is no longer required too.

Cleanup build.gradle of binary-compatibility-validator.

libraries/stdlib/jre7/build.gradle

@@ -53,7 +53,6 @@ compileKotlin {
             "-Xallow-kotlin-package",
             "-Xmultifile-parts-inherit",
             "-Xnormalize-constructor-calls=enable",
-            "-Xdump-declarations-to=${buildDir}/stdlib-jre7-declarations.json",
             "-module-name", project.name
     ]
 }