Remove unused rule

We are not generating any *.pyi files, this was an artifact from the
grpcio times.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

MANIFEST.in

@@ -13,4 +13,3 @@ recursive-exclude tests *
 recursive-include frequenz-api-microgrid/proto *.proto
 recursive-include frequenz-api-microgrid/submodules/frequenz-api-common/proto *.proto
 recursive-include frequenz-api-microgrid/submodules/api-common-protos *.proto
-recursive-include src *.pyi