poky-agl: remove outdated workarounds

Remove old workarounds from the poky-agl distro configuration that
no longer seem required:
- cmake DIRFILES tweak (SPEC-888). This actually seems to break
  cmake-native packaging during crosssdk building with scarthgap.
- do_package_write_rpm task vardepsexclude tweak (SPEC-1629).
  Based on some local tests with sstate, the original issue seems
  to have been fixed.

Bug-AGL: SPEC-4578

Change-Id: I898da1113ce74c5b899c9c943224598b0cc68289
Signed-off-by: Scott Murray <scott.murray@konsulko.com>