runtime: remove sizeToClass
runtime: remove sizeToClass

CL 32219 added precomputed sizeclass tables.

Remove the unused sizeToClass method which was previously only
called from initSizes.

Change-Id: I907bf9ed78430ecfaabbec7fca77ef2375010081
Reviewed-on: https://go-review.googlesource.com/38113
Run-TryBot: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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