llama_cpp server: prompt is a string

Not sure why this union type was here but taking a look at llama.py, prompt is only ever processed as a string for completion

This was breaking types when generating an openapi client

llama_cpp/server/app.py

@@ -126,7 +126,7 @@ def get_llama():
 )
 
 class CreateCompletionRequest(BaseModel):
-    prompt: Union[str, List[str]] = Field(
+    prompt: Optional[str] = Field(
         default="",
         description="The prompt to generate completions for."
     )
@@ -175,9 +175,6 @@ class Config:
 def create_completion(
     request: CreateCompletionRequest, llama: llama_cpp.Llama = Depends(get_llama)
 ):
-    if isinstance(request.prompt, list):
-        request.prompt = "".join(request.prompt)
-
     completion_or_chunks = llama(
         **request.dict(
             exclude={