Remove "performance hack" for the user matching

Where if the user input list has a single entry, we can return it.
If we have one user input and multiple trips, we can't just return that one
input. That is the input for one trip, not the others!
e-mission/e-mission-docs#727 (comment)

wrt performance, we already handled the major performance issue by only having
to manually match unprocessed entries after the end of the pipeline

Testing done:
- loaded a day with a single user input
- before this change, all trips were flagged with that input
- after this change, only the first was

www/js/survey/input-matcher.js

@@ -14,17 +14,11 @@ angular.module('emission.survey.inputmatcher', ['emission.plugin.logger'])
   }
 
   im.getUserInputForTrip = function(trip, nextTrip, userInputList) {
-    // If there is only one item in the list, return it.
-    // This make it compatible when fake list is given (for Enketo Survey).
-    // As well as optimize the performance.
     if (userInputList === undefined) {
         Logger.log("In getUserInputForTrip, no user input, returning []");
         return undefined;
     }
 
-    if (userInputList.length === 1) {
-      return userInputList[0];
-    }
     if (userInputList.length < 20) {
         console.log("Input list = "+userInputList.map(printUserInput));
     }