[FIX][SHELL] Modify PATH at login only

It's cleaner to only export my modified PATH when I'm logging in,
instead of doing it at each shell launch.

Because Zsh and Bash don't use the same mechanism to let you know you
are in a login session, you gotta check both separately. But that
doesn't matter because lightdm doesn't launch `sh` as a login shell
anyway... So the condition is super messy.

bash/.bashrc

@@ -55,9 +55,6 @@ source ~/.profile
 # Import my prompt
 source ~/.bash_prompt
 
-# set PATH so it includes user's private bin directories
-PATH="$HOME/bin:$HOME/.local/bin:$PATH"
-
 # Export our directory to Termite for opening new terminals
 if [[ $TERM == xterm-termite ]]; then
   . /etc/profile.d/vte.sh