fix(lib): remove `kubectx` stub prompt function from lib

Fixes #9974

lib/prompt_info_functions.zsh

@@ -19,7 +19,6 @@ function chruby_prompt_info \
   virtualenv_prompt_info \
   jenv_prompt_info \
   tf_prompt_info \
-  kubectx_prompt_info \
 {
   return 1
 }