Revert "Add headers to headermap action inputs"

[luispadron]

Reverts #888

[mattrobmattrob]

Why the revert of this one?

[justinseanmartin]

This seems right to me. After discussing with @congt - we don't think the hmaptool code requires the headers themselves to generate the hmap, just the paths to those files, therefore we don't need to declare them as inputs.

We could potentially make this a bit more explicit by having the input to _make_hmap from _make_headermap_impl be an array of strings instead of an array of files, but that isn't entirely necessary. It would make things more consistent with what we're passing internally to the make_hmap method and what the method generally requires to invoke the hmap generator (and we rely on being able to pass strings in the case of VFS overlays). That would technically be a breaking change though, because make_hmap is exposed publicly.

[luispadron]

Right I was under the impression the hmap tool would need to access the headers themelves but effectively what it does is create a file that looks like:

Foo/MyHeader.h -> MyHeader.h

[luispadron]

This API should 100% be string_list instead of label_list: https://github.com/bazel-ios/rules_ios/pull/893/files#diff-c7a846a20e1dd3e0a809cc965ea0d4134555adb5bc82fcc298bfb6a933f5e3c0R110