Vim: CTRLP should no longer use ag

According to the documentation for CTRLP, Vim's 'wildignore' and CTRLP's
g:ctrlp_custom_ignore don't apply if g:ctrlp_user_command is set.

Right now I don't feel like tweaking the 'ag' command to include the
ignores from 'wildignore' and such, so I'll just remove it altogether.