Revert "Refactor dep rewriting logic in github shims"

Summary:
X-link: facebook/buck2#782

Broke oss buck2 CI. buck2 does some path remapping that was not tested against
in the initial diff. The right way to handle this looks complex and will take
some time to implement, so reverting the changes in the interim.

Original commit changeset: 2cdf2fc1cbce

Original Phabricator Diff: D62896839

Reviewed By: zpao

Differential Revision: D63152891

fbshipit-source-id: 53aad436fdca15bece88b5ad0c12ea11390fb3f8

third-party/folly/src/.buckconfig

@@ -13,14 +13,9 @@ fbsource = shim
 fbcode_macros = shim
 buck = none
 bazel_skylib = shim
-folly = root
 
 [external_cells]
 prelude = bundled
 
 [parser]
 target_platform_detector_spec = target:root//...->prelude//platforms:default
-
-[oss]
-original_cell = fbcode
-mapped_dirs = folly