Unbreak build with Rust 1.45.0

error: options `-C embed-bitcode=no` and `-C lto` are incompatible

--- config/makefiles/rust.mk.orig	2020-06-22 22:55:02 UTC
+++ config/makefiles/rust.mk
@@ -47,7 +47,7 @@ cargo_rustc_flags = $(CARGO_RUSTCFLAGS)
 ifndef DEVELOPER_OPTIONS
 ifndef MOZ_DEBUG_RUST
 # Enable link-time optimization for release builds.
-cargo_rustc_flags += -C lto
+cargo_rustc_flags += 
 endif
 endif