Remove redundant type args in briar-headless/build.gradle

In briar-headless/build.gradle in the place where the entries of the fat jar get sorted, I stumbled upon some redundant type arguments. It not a big deal, but the TreeMap<> doesn't need to repeat <String, JarEntry> from Map<String, JarEntry>. In regular Java code, Android Studio would warn about it, but apparently linting in Gradle build files either has the check for that disabled or doesn't work that well there.