Simplify the thunk targets since we don't support apps as components

Since we don't support using the component build to produce mojo apps,
we can simplify the build targets in a few ways:

*) every mojo_native_application must depend on the c system thunks,
so just make that part of the template instead of requiring the dep
*) there's no such thing as depending on gles2 headers from a component,
so delete the forwarding group.

Most targets that want to use the gles2 headers in a mojo context
want to depend on an implementation through the thunks, so
//mojo/public/c/gles2 does just that. A smaller number of targets (such
as the implementation of the thunks) want to just depend on the headers
but not an impl, so they can depend on //mojo/public/c/gles2:headers.
The //mojo/public/gles target isn't that useful since the only thing we
expose is a set of C entry points.

We can probably also simplify the c system targets, but that's trickier
due to more extensive use from the chromium side.

BUG=438701
R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/780733002