Java: Remove snapshotDate from dbscheme

It's not been used for some time.

Author: igfoo

java/ql/lib/config/semmlecode.dbscheme

@@ -202,10 +202,6 @@ externalData(
   string value : string ref
 );
 
-snapshotDate(
-  unique date snapshotDate : date ref
-);
-
 sourceLocationPrefix(
   string prefix : string ref
 );

java/ql/lib/config/semmlecode.dbscheme.stats

@@ -3965,17 +3965,6 @@
                 </dep>
             </dependencies>
         </relation>
-        <relation>
-            <name>snapshotDate</name>
-            <cardinality>1</cardinality>
-            <columnsizes>
-                <e>
-                    <k>snapshotDate</k>
-                    <v>1</v>
-                </e>
-            </columnsizes>
-            <dependencies/>
-        </relation>
         <relation>
             <name>sourceLocationPrefix</name>
             <cardinality>569</cardinality>