build: remove rm dreg

Undo CL 4254056 now that enough time has elapsed
that people with old trees have all built and removed
those files (which are no longer generated in that location).

Fixes #1581.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/4802059

src/Make.pkg

@@ -59,7 +59,7 @@ coverage:
 	gotest
 	6cov -g $(shell pwd) $O.out | grep -v '_test\.go:'
 
-CLEANFILES+=*.so _obj _test _testmain.go *.exe _cgo* *.cgo[12].* test.out build.out
+CLEANFILES+=*.so _obj _test _testmain.go *.exe _cgo* test.out build.out
 
 test:
 	gotest