Remove unused config.h header from umbrella header directory

The file include/AblyDeltaCodec.modulemap contains an umbrella header
declaration:

> umbrella header "AblyDeltaCodec/AblyDeltaCodec.h"

According to Clang’s modules documentation
(https://clang.llvm.org/docs/Modules.html):

> An umbrella header includes all of the headers within its directory

> Use the -Wincomplete-umbrella warning option to ask Clang to complain
> about headers not covered by the umbrella header or the module map.

-Wincomplete-umbrella is enabled by default
(https://clang.llvm.org/docs/DiagnosticsReference.html#wincomplete-umbrella).

include/AblyDeltaCodec/AblyDeltaCodec.h doesn’t contain an import of its
neighbouring file config.h (a symlink to a header used internally),
which hence triggers the above warning ("umbrella header for module
'AblyDeltaCodec' does not include header 'config.h'") in apps importing
the AblyDeltaCodec module, i.e. apps importing the AblyDeltaCodec SPM
package.

That config.h symlink is _correctly_ not included in the umbrella header
since it is not part of the public interface of this module. Hence the
correct thing to do is to delete it.

Closes #15.