Remove folly/gen:base from folly/gen:gen
Remove folly/gen:base from folly/gen:gen

Summary: Leaving `folly/gen:gen` empty. It will be removed later.

Reviewed By: yfeldblum

Differential Revision: D5761720

fbshipit-source-id: 7763b8cd0564b3e4eeb7fdb91f71e55d19f9fcf9

facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro,facebook/bistro