hgignore: cleanup obsolete entries.

R=golang-dev, dave, r
CC=golang-dev
https://golang.org/cl/9935043

.hgignore

@@ -32,7 +32,6 @@ misc/dashboard/builder/builder
 misc/goplay/goplay
 misc/osx/*.pkg
 misc/osx/*.dmg
-src/cmd/6a/6a
 src/cmd/?a/y.output
 src/cmd/?l/enam.c
 src/cmd/cc/y.output
@@ -40,10 +39,6 @@ src/cmd/dist/dist.dSYM
 src/cmd/gc/mkbuiltin1
 src/cmd/gc/opnames.h
 src/cmd/gc/y.output
-src/pkg/exp/norm/maketables
-src/pkg/exp/norm/maketesttables
-src/pkg/exp/norm/normregtest
-src/pkg/exp/ebnflint/ebnflint
 src/pkg/go/doc/headscan
 src/pkg/runtime/goc2c
 src/pkg/runtime/mkversion