cmd/internal/obj: eliminate unnecessary ctxt.Cursym assignment
cmd/internal/obj: eliminate unnecessary ctxt.Cursym assignment

None of the following code uses it.

Passes toolstash-check -all.

Updates #15756

Change-Id: Ieeaaca8ba31e5c345c0c8a758d520b24be88e173
Reviewed-on: https://go-review.googlesource.com/38444
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>

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