cmd/compile/internal/syntax: remove some outdated comments (cleanup)
cmd/compile/internal/syntax: remove some outdated comments (cleanup)

Change-Id: If242bb99d501420827b764c908580f2363e01ac4
Reviewed-on: https://go-review.googlesource.com/69730
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

josharian/go,codestation/go,nawawi/go,josharian/go,akutz/go,josharian/go,codestation/go,polyverse-security/go,akutz/go,codestation/go,CAFxX/go,polyverse-security/go,nawawi/go,codestation/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,akutz/go,cloudflare/go,golang/go,cloudflare/go,golang/go,golang/go,Cofyc/go,polyverse-security/go,golang/go,cloudflare/go,cloudflare/go,josharian/go,Cofyc/go,akutz/go,cloudflare/go,Cofyc/go,josharian/go,polyverse-security/go,josharian/go,golang/go,josharian/go,golang/go,CAFxX/go,Cofyc/go,polyverse-security/go,nawawi/go,cloudflare/go,polyverse-security/go,nawawi/go,codestation/go,josharian/go,Cofyc/go,polyverse-security/go,CAFxX/go,Cofyc/go,polyverse-security/go,akutz/go,christopher-henderson/Go,nawawi/go,CAFxX/go,CAFxX/go,codestation/go,nawawi/go,Cofyc/go,cloudflare/go,akutz/go,nawawi/go,codestation/go,CAFxX/go,akutz/go,Cofyc/go,akutz/go,codestation/go