Delete prompt_powerlevel9k_setup

I think this should have been removed as part of 0a07ba27a2.

Currently I'm seeing this warning after updating to `master`:
```
Couldn't read file /Users/jeffwidman/.zprezto/modules/prompt/functions/prompt_powerlevel9k_setup containing theme powerlevel9k.
```
main
Jeff Widman 4 years ago
parent f651140f2c
commit 63bc5061a7

@ -1 +0,0 @@
../external/powerlevel9k/powerlevel9k.zsh-theme
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