cmd/compile: delete unused Convconst function
cmd/compile: delete unused Convconst function

Convconst is not used in the new backend, and all its callers
were deleted in CL 29168 (cmd/compile: delete lots of the legacy
backend). iconv was an helper function for Convconst.

Updates #16357

Change-Id: I65c7345586d7af81cdc2fb09c68f744ffb161a17
Reviewed-on: https://go-review.googlesource.com/30090
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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