Simplify format strings check

In most cases underlaying implementation does not properly handle when
some format string is omitted from the string, so let users override the
cases where it works.

trans/checks/format.py

@@ -132,19 +132,8 @@ class BaseFormatCheck(TargetCheck):
             0,
             len(sources) > 1
         )
-
         if singular_check:
-            if len(sources) == 1:
-                return True
-            plural_check = self.check_format(
-                sources[1],
-                targets[0],
-                unit,
-                1,
-                True
-            )
-            if plural_check:
-                return True
+            return True
 
         # Do we have more to check?
         if len(sources) == 1: