z plugin: fix loading from custom location

The old implementation would attempt to load both the default and custom
implementations, with the custom one coming first, so it would get clobbered
by the default version.

plugins/z/z.plugin.zsh

-_load_z() {
-  source $1/z.sh
-}
-
-[[ -f $ZSH_CUSTOM/plugins/z/z.plugin.zsh ]] && _load_z $ZSH_CUSTOM/plugins/z
-[[ -f $ZSH/plugins/z/z.plugin.zsh ]] && _load_z $ZSH/plugins/z
+source "${0:h}/z.sh"