.gitignore: remove obselete entries

Change-Id: I520a0f2c2d14f744a5d2e8cf93963eec1cf3e9ba
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/6732Reviewed-by: Andrew Gerrand <adg@golang.org>

.gitignore

@@ -21,22 +21,14 @@ _testmain.go
 build.out
 test.out
 doc/articles/wiki/*.bin
-include/plan9/libc_plan9.h
 misc/cgo/life/run.out
 misc/cgo/stdio/run.out
 misc/cgo/testso/main
-misc/dashboard/builder/builder
-src/liblink/anames?.c
 src/cmd/*/y.output
 src/cmd/cgo/zdefaultcc.go
-src/cmd/dist/dist.dSYM
-src/cmd/gc/mkbuiltin1
-src/cmd/gc/opnames.h
 src/cmd/go/zdefaultcc.go
 src/cmd/internal/obj/zbootstrap.go
 src/go/doc/headscan
-src/runtime/mkversion
-src/runtime/zaexperiment.h
 src/runtime/zversion.go
 src/unicode/maketables
 src/*.*/