fix usage of 'typeset -U'

From man zsh:
for variables with shared values  it is  therefore  recommended to set the flag
for all interfaces, e.g. `typeset -U PATH path'.

Otherwise one of them is not unique:
% echo ${parameters[path]}
array-unique-special
% echo ${parameters[PATH]}
scalar-export-special

After:
% echo ${parameters[path]}
array-unique-special
% echo ${parameters[PATH]}
scalar-export-unique-special

Additionally drop a superflous for loop.