prompt: remove fallback for $fish_color_host

This was mostly dead, since $fish_color_host is set to normal in
__fish_config_interactive. The assignment was only used if the user
explicitly unsets fish_color_host (which they shouldn't, really).
Anyway it's weird to use cyan, use normal instead.
[ci skip]

share/tools/web_config/sample_prompts/classic_vcs.fish

@@ -6,10 +6,6 @@ function fish_prompt --description 'Write out the prompt'
     set -l last_status $status
     set -l normal (set_color normal)
 
-    # Initialize our variables.
-    set -q fish_color_host
-    or set -U fish_color_host -o cyan
-
     # Color the prompt differently when we're root
     set -l color_cwd $fish_color_cwd
     set -l prefix