remove incorrect dependency on librustc_front

Author: nikomatsakis

src/librustc_incremental/Cargo.toml

@@ -13,5 +13,4 @@ graphviz = { path = "../libgraphviz" }
 rbml = { path = "../librbml" }
 rustc = { path = "../librustc" }
 rustc_data_structures = { path = "../librustc_data_structures" }
-rustc_front = { path = "../librustc_front" }
 serialize = { path = "../libserialize" }