refactor: 중복 되는 메서드 제거

1. getLine 메서드를 LineRepository 클래스의 동일한 기능을 하는 메서드로 교체

Author: jujubebat

src/main/java/subway/domain/selector/sectionitem/AddSectionItem.java

@@ -38,25 +38,14 @@ public void execute() {
     }
 
     private void addStation(String lineName, String stationName, int order) {
-        Line line = getLine(lineName);
+        Line line = LineRepository.getLineByName(lineName);
         List<Station> stations = line.stations();
         sectionValidator.validateSectionOrder(order, stations.size());
         Station station = new Station(stationName);
         sectionValidator.validateStationsDuplication(line, station);
         line.addStationByIndex(station, order - 1);
     }
 
-    private Line getLine(String lineName) {
-        List<Line> lines = LineRepository.lines();
-
-        for (Line line : lines) {
-            if (line.getName().equals(lineName)) {
-                return line;
-            }
-        }
-        return null;
-    }
-
 }