Small code improvements

Removed some non-translatable strings and just hardcoded them in the code, like it's being done for other string separators. This also deduplicates some code by using Localization.
Used some Kotlin feature to reduce code.

app/src/main/java/org/schabi/newpipe/util/PicassoHelper.java

@@ -81,10 +81,6 @@ public final class PicassoHelper {
         picassoInstance.cancelTag(tag);
     }
 
-    public static void cancelRequest(final Target target) {
-        picassoInstance.cancelRequest(target);
-    }
-
     public static void setIndicatorsEnabled(final boolean enabled) {
         picassoInstance.setIndicatorsEnabled(enabled); // useful for debugging
     }