Delete header information from ObjcProvider

[googlewalt]

In #10674 we had migrated the header information used for compilation from ObjcProvider to CcInfo, but we still left the direct header information in ObjcProvider, primarily for the use of our internal IDEs. But CcInfo has direct header information also, so the ObjcProvider information is largely redundant, and users should migrate to the direct header APIs in CcCompilationContext.

We plan to delete the direct header information in ObjcProvider. This will also allow us to delete some cumbersome APIs that have remained just to support that. ObjcProvider's direct_sources will remain, and continue to contain headers from the srcs attribute.

Starlark rules should be migrated for this change as follows:

1. Migrate uses of ObjcProvider's direct_headers to CcCompilationContext. ObjcProvider's direct_headers corresponds to CcCompilationContext's direct_public_headers plus direct_textual_headers. Related, CcCompilationContext also provides direct_private_headers and direct_headers.

2. Delete any header parameters to apple_common.new_objc_provider().