Revert "Add template dependency to Mantle extension"

This broke beta labs and would have broken all production
wikis tomorrow. There is no logic in wmf-config saying
Mantle needs to be enabled on wikis with MobileFrontend,
right now it's only tied to Flow.

This reverts commit df2f2e1ace788c3b5f3652ccf2228ebd0d9f5ebc.

Change-Id: I9f0f5db44a623560de8008f7c3966a433ada0729
Merge "Revert "Add template dependency to Mantle extension""