[split] Move brew plugin to oh-my-fish/plugin-brew

https://github.com/oh-my-fish/plugin-brew

plugins/brew/brew.load

-set -l brew_paths /usr/local/bin /usr/bin /bin /usr/local/sbin /usr/sbin /sbin
-
-# Append all existing brew paths to PATH
-set -l existing_brew_paths
-for brew_path in $brew_paths
-  if test -d $brew_path
-    set PATH $PATH $brew_path
-    set existing_brew_paths $existing_brew_paths $brew_path
-  end
-end
-
-# Remove brew paths from tail to head that were not recently added
-set -l number_of_paths_to_ignore (math (count $PATH) - (count $existing_brew_paths))
-for i in (seq (count $PATH))[-1..1]
-  if test $i -le $number_of_paths_to_ignore
-    if contains $PATH[$i] $brew_paths
-      set -e PATH[$i]
-    end
-  end
-end
-
-set -e brew_paths
-set -e existing_brew_paths
-set -e existing_brew_paths_count