Re-enable Checkstyle

Also disable UnnecessaryParentheses due to false positive.

pom.xml

@@ -125,8 +125,7 @@
           <headerLocation>src/main/resources/copyright_header.txt</headerLocation>
           <includeTestSourceDirectory>true</includeTestSourceDirectory>
           <violationSeverity>warning</violationSeverity>
-          <!-- TODO: remove once source fixed -->
-          <failOnViolation>false</failOnViolation>
+          <failOnViolation>true</failOnViolation>
         </configuration>
         <dependencies>
           <dependency>

src/main/resources/checkstyle.xml

@@ -118,7 +118,6 @@
     </module>
     <module name="TypecastParenPad"/>
     <module name="TypeName"/>
-    <module name="UnnecessaryParentheses"/>
     <module name="UnusedImports"/>
     <module name="UpperEll"/>
     <module name="VisibilityModifier">