language_model: Remove dependencies on individual model provider crates (#25503)

This PR removes the dependencies on the individual model provider crates
from the `language_model` crate.

The various conversion methods for converting a `LanguageModelRequest`
into its provider-specific request type have been inlined into the
various provider modules in the `language_models` crate.

The model providers we provide via Zed's cloud offering get to stay, for
now.

Release Notes:

- N/A

crates/language_model/src/role.rs

@@ -45,43 +45,3 @@ impl Display for Role {
         }
     }
 }
-
-impl From<Role> for ollama::Role {
-    fn from(val: Role) -> Self {
-        match val {
-            Role::User => ollama::Role::User,
-            Role::Assistant => ollama::Role::Assistant,
-            Role::System => ollama::Role::System,
-        }
-    }
-}
-
-impl From<Role> for open_ai::Role {
-    fn from(val: Role) -> Self {
-        match val {
-            Role::User => open_ai::Role::User,
-            Role::Assistant => open_ai::Role::Assistant,
-            Role::System => open_ai::Role::System,
-        }
-    }
-}
-
-impl From<Role> for deepseek::Role {
-    fn from(val: Role) -> Self {
-        match val {
-            Role::User => deepseek::Role::User,
-            Role::Assistant => deepseek::Role::Assistant,
-            Role::System => deepseek::Role::System,
-        }
-    }
-}
-
-impl From<Role> for lmstudio::Role {
-    fn from(val: Role) -> Self {
-        match val {
-            Role::User => lmstudio::Role::User,
-            Role::Assistant => lmstudio::Role::Assistant,
-            Role::System => lmstudio::Role::System,
-        }
-    }
-}