Fixed: NetBeforeOverheadReport.groovy does not compile (OFBIZ-12958)

First note that the Birt component is currently disabled because of
eclipse-birt/birt#625
To test OFBIZ-12957 I enabled it and got this incomprehensible error:

> Task :compileGroovy
startup failed:
C:\projectsASF\Git\ofbiz-framework\plugins\birt\src\main\groovy\org\apache\ofbiz\birt\order\NetBeforeOverheadReport.groovy: 21: unable to resolve class org.apache.ofbiz.base.util.UtilDatetime
 @ line 21, column 1.
   import org.apache.ofbiz.base.util.UtilDatetime
   ^
1 error
> Task :compileGroovy FAILED
FAILURE: Build failed with an exception.

This fixes it (still incomprehensible, it's disabled anyway)
Also import java.sql.* is unused

birt/src/main/groovy/org/apache/ofbiz/birt/order/NetBeforeOverheadReport.groovy

@@ -18,11 +18,8 @@
 */
 package org.apache.ofbiz.birt.order
 
-import org.apache.ofbiz.base.util.UtilDatetime
-import java.sql.*
-
 productStoreId = parameters.productStoreId
-DateTime = UtilDateTime.nowTimestamp()
+DateTime = org.apache.ofbiz.base.util.UtilDateTime.nowTimestamp()
 String DateStr = DateTime
 DateDay = DateStr.substring(0,10)
 DateMonth = DateStr.substring(5,7)