build: remove unnecessary references to GOBIN and GOROOT

All scripts and makefiles assume that GOBIN is correctly set
in PATH.

R=rsc
CC=golang-dev
https://golang.org/cl/2043041
diff --git a/src/clean.bash b/src/clean.bash
index a62f339..8f34310 100755
--- a/src/clean.bash
+++ b/src/clean.bash
@@ -27,6 +27,6 @@
 	if test -f clean.bash; then
 		bash clean.bash
 	else
-		"$GOBIN"/gomake clean
+		gomake clean
 	fi
 )done