strings: improve readability of IndexAny and LastIndexAny functions.
strings: improve readability of IndexAny and LastIndexAny functions.

This change removes the check of len(chars) > 0 inside the Index and
IndexAny functions which was redundant.

Change-Id: Iffbc0f2b3332c6e31c7514b5f644b6fe7bdcfe0d
Reviewed-on: https://go-review.googlesource.com/65910
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

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