cmd/compile: delete bitset16
cmd/compile: delete bitset16

It is no longer used.

Change-Id: Id64f387867a0503d13eaecda12e6606682c24595
Reviewed-on: https://go-review.googlesource.com/41790
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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