Remove Atom plugin

The Atom plugin is obsolete.

See #3193

plugins/atom/atom.plugin.zsh

@@ -1,37 +0,0 @@
-case $OSTYPE in
-darwin*)
-    local _atom_paths > /dev/null 2>&1
-    _atom_paths=(
-        "$HOME/Applications/Atom.app"
-        "/Applications/Atom.app"
-    )
-
-    for _atom_path in $_atom_paths; do
-        if [[ -a $_atom_path ]]; then
-            alias at="open -a '$_atom_path'"
-            break
-        fi
-    done
-    ;;
-cygwin)
-    local _atom_path > /dev/null 2>&1
-
-    _atom_path=${LOCALAPPDATA}/atom/bin/atom
-
-    if [[ -a $_atom_path ]]; then
-        cyg_open_atom()
-        {
-            if [[ -n $1 ]]; then
-                ${_atom_path} `cygpath -w -a $1`
-            else
-                ${_atom_path}
-            fi
-        }
-
-        alias at=cyg_open_atom
-    fi
-    ;;
-linux*)
-    # Alerts the user if 'atom' is not a found command.
-    type atom >/dev/null 2>&1 && alias at="atom" || { echo >&2 "You have enabled the atom oh-my-zsh plugin on Linux, but atom is not a recognized command. Please make sure you have it installed before using this plugin."; }
-esac