remove some old (unused/duplicate) files (#3975)

sync/cov.sh

@@ -1,9 +0,0 @@
-if ! type kcov > /dev/null; then
-   	echo "Install kcov first (details inside this file). Aborting."
-	exit 1
-fi
-
-cargo test --no-run || exit $?
-mkdir -p target/coverage
-kcov --exclude-pattern ~/.multirust,rocksdb,secp256k1,sync/src/tests --include-pattern sync/src --verify target/coverage target/debug/ethsync*
-xdg-open target/coverage/index.html