Use a button instead of a link to remove a marker

Using a button for interactive elements is better, as explained in
commit 5311daadf.

Since buttons with "type=button" do nothing by default, we no longer
need to call `preventDefault()` when clicking it.

app/assets/stylesheets/map_location.scss

@@ -1,7 +1,8 @@
 .map-location-remove-marker {
   border-bottom: 1px dotted #cf2a0e;
+  border-radius: 0;
   color: $delete;
-  display: inline-block;
+  cursor: pointer;
   margin-bottom: $line-height;
   margin-top: $line-height / 2;
 
@@ -10,7 +11,6 @@
   &:focus {
     border-bottom-style: solid;
     color: #cf2a0e;
-    text-decoration: none;
   }
 }