syscall: remove unused {dragonfly,illumos,solaris}64Bit constants
syscall: remove unused {dragonfly,illumos,solaris}64Bit constants

These are unused since CL 153837. illumos64Bit was added by CL 174457
but was never used.

Change-Id: I34a1bd41cf70f8a07e57f93a71de3c6034fcaf7b
Reviewed-on: https://go-review.googlesource.com/c/go/+/175358
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

CAFxX/go,akutz/go,cloudflare/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,josharian/go,akutz/go,josharian/go,nawawi/go,akutz/go,josharian/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,josharian/go,akutz/go,CAFxX/go,josharian/go,golang/go,akutz/go,golang/go,golang/go,cloudflare/go,nawawi/go,josharian/go,CAFxX/go,CAFxX/go,nawawi/go,akutz/go,CAFxX/go,akutz/go,golang/go,cloudflare/go,nawawi/go,golang/go,golang/go,akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,josharian/go,cloudflare/go,josharian/go,cloudflare/go