Python: Always enable legacy taint tracking configuration

If the legacy configuration is only enabled if there are no other
configurations, defining a configuration in an imported library can lead to
unwanted results. For example, code that uses `any(MyTaintKind t).taints(node)`
would *stop* working, if it did not define its own configuration. (this actually
happened to us)

We performed a dist-compare to ensure there is not a performance deg ration by
doing this. Results at https://git.semmle.com/gist/rasmuswl/a1eca07f3a92f5f65ee78d733e5d260e

Tests that were affected by this:

- RockPaperScissors + Simple: new edges because no configuration was defined for
  SqlInjectionTaint or CommandInjectionTaint
- CleartextLogging + CleartextStorage: new edges because no configuration was
  defined before, AND duplicate deges.
- TestNode: new edges because no configuration was defined before

- PathInjection: Duplicate edges
- TarSlip: Duplicate edges
- CommandInjection: Duplicate edges
- ReflectedXss: Duplicate edges
- SqlInjection: Duplicate edges
- CodeInjection: Duplicate edges
- StackTraceExposure: Duplicate edges
- UnsafeDeserialization: Duplicate edges
- UrlRedirect: Duplicate edges

python/ql/src/semmle/python/dataflow/Legacy.qll

@@ -33,26 +33,18 @@ private class LegacyConfiguration extends TaintTracking::Configuration {
     }
 
     override predicate isSource(TaintSource src) {
-        isValid() and
         src = src
     }
 
     override predicate isSink(TaintSink sink) {
-        isValid() and
         sink = sink
     }
 
     override predicate isSanitizer(Sanitizer sanitizer) {
-        isValid() and
         sanitizer = sanitizer
     }
 
-    private predicate isValid() {
-        not exists(TaintTracking::Configuration config | config != this)
-    }
-
     override predicate isAdditionalFlowStep(DataFlow::Node src, DataFlow::Node dest) {
-        isValid() and
         exists(DataFlowExtension::DataFlowNode legacyExtension |
             src.asCfgNode() = legacyExtension
             |
@@ -67,7 +59,6 @@ private class LegacyConfiguration extends TaintTracking::Configuration {
     }
 
     override predicate isAdditionalFlowStep(DataFlow::Node src, DataFlow::Node dest, TaintKind srckind, TaintKind destkind) {
-        isValid() and
         exists(DataFlowExtension::DataFlowNode legacyExtension |
             src.asCfgNode() = legacyExtension
             |
@@ -76,7 +67,6 @@ private class LegacyConfiguration extends TaintTracking::Configuration {
     }
 
     override predicate isBarrierEdge(DataFlow::Node src, DataFlow::Node dest) {
-        isValid() and
         (
             exists(DataFlowExtension::DataFlowVariable legacyExtension |
                 src.asVariable() = legacyExtension and
@@ -91,4 +81,3 @@ private class LegacyConfiguration extends TaintTracking::Configuration {
     }
 
 }
-