Update checkstyle.xml

Removed "serverity" property so if checkstyle fails the build does not pass 
Module:
* NewlineAtEndOfFile
* JavadocType:
 * Checks for missing @param 
 * Checks for invalid tags
* JavadocStyle:
 * Checks if first sentence is an actual sentence 
 * If sentences contains the regex ([.?!][ \t\n\r\f<])|([.?!]$) at the end
* UnusedImports