Remove unnecessary mainClassName

Since each sample has (supposedly) only one application class and not
a lot of the code, the `mainClassName` attribute in the gradle build is
not really useful and can lead to inconsistency if the application class
is moved.

See gh-3588

spring-boot-samples/spring-boot-sample-web-ui/build.gradle

@@ -23,8 +23,6 @@ apply plugin: 'eclipse'
 apply plugin: 'idea'
 apply plugin: 'spring-boot'
 
-mainClassName = "sample.web.ui.SampleWebUiApplication"
-
 springBoot {
     classifier = 'exec'
 }