tests/run-make: Remove wrong blessing advice

run-make tests are not special but can be blessed like other tests, like
this:

    ./x.py test --bless tests/run-make/unknown-mod-stdin

Author: Enselic

tests/run-make/unknown-mod-stdin/Makefile

@@ -5,7 +5,6 @@ include ../tools.mk
 all:
 	echo 'mod unknown;' | $(RUSTC) --crate-type rlib - >$(TMPDIR)/unknown-mod.stdout 2>$(TMPDIR)/unknown-mod.stderr || echo "failed successfully"
 
-# Bless like this: RUSTC_BLESS_TEST=1 ./x.py test tests/run-make/unknown-mod-stdin
 ifdef RUSTC_BLESS_TEST
 	cp "$(TMPDIR)"/unknown-mod.stdout unknown-mod.stdout
 	cp "$(TMPDIR)"/unknown-mod.stderr unknown-mod.stderr