more SpotBugs related / inspired code clean up (see FINERACT-702)

based upon code review of changes made by @awasum in previous commit:

 - fix clear contains() GC_UNRELATED_TYPES bug in Loan

 - fix clear equals() EC_UNRELATED_TYPES bug in Loan

 - fix more equals() & hashCode() in...
    * Charge
    * TrialBalance
    * LoanProductBorrowerCycleVariations
    * LoanProductProvisioningEntry

 - fix AccountingConstants to avoid using @FindBugsSuppressWarnings

 - fix SelfLoansDataValidator instead of @FindBugsSuppressWarnings

 - minor clean up in build.gradle (spotbugs has replaced findbugs)

PS: This change originally included also fixing the missing hashCode()
    and equals() in ShareAccountCharge, but that turned out to be non
    trivial, see FINERACT-827, and is thus not yet fixed by this.

fineract-provider/build.gradle

@@ -64,7 +64,6 @@ apply plugin: 'openjpa'
 apply plugin: "com.github.spotbugs"
 
 // apply plugin: 'pmd'
-// apply plugin: 'findbugs'
 
 dependencyManagement {
   imports {
@@ -341,11 +340,6 @@ compileJava{
 /*
 pmd {
     sourceSets = [sourceSets.main]
-    ignoreFailures = true
-}
-findbugs {
-    ignoreFailures = true
-    sourceSets = [sourceSets.main]
 }
 */