Remove vendor substrate (polkadot-evm#115)

* remove Substrate as submodule, use git reference directly instead

* add instruction

* impl Clone for FrontierBlockImport

* remove evm as well

* use frontier branch

* update with latest master credits to @crystalin

.gitmodules

@@ -1,9 +1,3 @@
 [submodule "vendor/ethereum"]
 	path = vendor/ethereum
 	url = https://github.com/rust-blockchain/ethereum
-[submodule "vendor/substrate"]
-	path = vendor/substrate
-	url = https://github.com/paritytech/substrate
-[submodule "vendor/evm"]
-	path = vendor/evm
-	url = https://github.com/rust-blockchain/evm