Remove mstch build from mcrouter OSS build
Remove mstch build from mcrouter OSS build

Summary: Thrift now maintains its own fork of the `mstch` library in `thrift/compiler/mustache/`. So downloading and building `mstch` separately is no longer needed as part of mcrouter's build.

Reviewed By: vitaut

Differential Revision: D15283782

fbshipit-source-id: 030de560b83369b640796d9fce9138f6faf95752

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter