Rm check on dependency in the C# reflection API (protocolbuffers#2051)

This check adds a few constraints on the way to build a project when we have
a proto file which imports another one. In particular, on projects which
build both C# and Java, it's easy to end up with exceptions like

    Expected: included.proto but was src/main/protobuf/included.proto

A user may work around this issue, but it may add unnecessary constraints
on the layout of the project.

According to protocolbuffers@f3504cf#diff-ecb0b909ed572381a1c8d1994f09a948R309
it has already been considered to get rid of this check, for
similar considerations, and because it doesn't exist in the Java code

csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs

@@ -251,17 +251,6 @@ private static FileDescriptor BuildFrom(ByteString descriptorData, FileDescripto
                     "Dependencies passed to FileDescriptor.BuildFrom() don't match " +
                     "those listed in the FileDescriptorProto.");
             }
-            for (int i = 0; i < proto.Dependency.Count; i++)
-            {
-                if (dependencies[i].Name != proto.Dependency[i])
-                {
-                    throw new DescriptorValidationException(
-                        result,
-                        "Dependencies passed to FileDescriptor.BuildFrom() don't match " +
-                        "those listed in the FileDescriptorProto. Expected: " +
-                        proto.Dependency[i] + " but was: " + dependencies[i].Name);
-                }
-            }
 
             result.CrossLink();
             return result;
@@ -341,4 +330,4 @@ public override string ToString()
         /// </value>
         public static FileDescriptor DescriptorProtoFileDescriptor { get { return DescriptorReflection.Descriptor; } }
     }
-}
+}