gitignore: remove obsolete reference to .hgignore in comment
gitignore: remove obsolete reference to .hgignore in comment

Change-Id: Id56bd12269c24fd29c83d17bc61e5189713ffee9
Reviewed-on: https://go-review.googlesource.com/c/sys/+/224337
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/sys,multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys