fix(completion): --flag=val breaks zsh completion

This is a bug I ran into when working on Helm completion.
I was surprised that it didn't happen when I was using
kubectl, so I investigated and found a PR that fixed this
bug in kubectl:
https://github.com/kubernetes/kubernetes/pull/48553

I duplicated the code in this commit which:

Removes __helm_declare, which is safe to do since
`declare -F` is already replaced to `whence -w` by
__helm_convert_bash_to_zsh().

The problem was that calling "declare" from inside a function
scopes the declaration to that function only.  So "declare"
should not be called through __helm_declare() but instead
directly.

To reproduce:
1- setup helm completion in zsh
2- helm --kubeconfig=$HOME/.kube/config statu<TAB>

you will get the error:
__helm_handle_flag:27: bad math expression: operand expected at end of string

Co-authored-by: Kazuki Suda <kazuki.suda@gmail.com>
Signed-off-by: Marc Khouzam <marc.khouzam@ville.montreal.qc.ca>

cmd/helm/completion.go

@@ -126,13 +126,6 @@ __helm_compgen() {
 __helm_compopt() {
 	true # don't do anything. Not supported by bashcompinit in zsh
 }
-__helm_declare() {
-	if [ "$1" == "-F" ]; then
-		whence -w "$@"
-	else
-		builtin declare "$@"
-	fi
-}
 __helm_ltrim_colon_completions()
 {
 	if [[ "$1" == *:* && "$COMP_WORDBREAKS" == *:* ]]; then
@@ -210,7 +203,7 @@ __helm_convert_bash_to_zsh() {
 	-e "s/${LWORD}__ltrim_colon_completions${RWORD}/__helm_ltrim_colon_completions/g" \
 	-e "s/${LWORD}compgen${RWORD}/__helm_compgen/g" \
 	-e "s/${LWORD}compopt${RWORD}/__helm_compopt/g" \
-	-e "s/${LWORD}declare${RWORD}/__helm_declare/g" \
+	-e "s/${LWORD}declare${RWORD}/builtin declare/g" \
 	-e "s/\\\$(type${RWORD}/\$(__helm_type/g" \
 	-e 's/aliashash\["\(.\{1,\}\)"\]/aliashash[\1]/g' \
 	<<'BASH_COMPLETION_EOF'