Refactor zshrc. Drop zshenv

Problem: path_helper(8) interferes adds records specified in /etc/paths, thus
/usr/local/bin priority decreases and brew-installed packages that have
local alternatives (e.g. vim) do not get loaded.

Solution: use zshrc as single $PATH setup place.

Signed-off-by: Aleksandrs Ļedovskis <aleksandrs@ledovskis.lv>