Don't #import the .m files.

As bazel folks are looking at getting auto generation of module maps going and
the importing of sources files causes issues there.  We were only do it to
hack around some of the apple linker behaviors around objc classes and
categories, but even that isn't complete and CocoaPods was already doing -ObjC,
and developers not using pods could have still needed it to ensure everything
was linked anyways; so drop the hack of importing sources.
diff --git a/Protobuf.podspec b/Protobuf.podspec
index 75a5e31..2523076 100644
--- a/Protobuf.podspec
+++ b/Protobuf.podspec
@@ -18,18 +18,14 @@
   s.source_files = 'objectivec/*.{h,m}',
                    'objectivec/google/protobuf/Any.pbobjc.{h,m}',
                    'objectivec/google/protobuf/Api.pbobjc.{h,m}',
-                   'objectivec/google/protobuf/Duration.pbobjc.h',
+                   'objectivec/google/protobuf/Duration.pbobjc.{h,m}',
                    'objectivec/google/protobuf/Empty.pbobjc.{h,m}',
                    'objectivec/google/protobuf/FieldMask.pbobjc.{h,m}',
                    'objectivec/google/protobuf/SourceContext.pbobjc.{h,m}',
                    'objectivec/google/protobuf/Struct.pbobjc.{h,m}',
-                   'objectivec/google/protobuf/Timestamp.pbobjc.h',
+                   'objectivec/google/protobuf/Timestamp.pbobjc.{h,m}',
                    'objectivec/google/protobuf/Type.pbobjc.{h,m}',
                    'objectivec/google/protobuf/Wrappers.pbobjc.{h,m}'
-  # Timestamp.pbobjc.m and Duration.pbobjc.m are #imported by GPBWellKnownTypes.m. So we can't
-  # compile them (duplicate symbols), but we need them available for the importing:
-  s.preserve_paths = 'objectivec/google/protobuf/Duration.pbobjc.m',
-                     'objectivec/google/protobuf/Timestamp.pbobjc.m'
   # The following would cause duplicate symbol definitions. GPBProtocolBuffers is expected to be
   # left out, as it's an umbrella implementation file.
   s.exclude_files = 'objectivec/GPBProtocolBuffers.m'