Remove an unused field in Diagnostic from zed.proto (#27091)

Release Notes:

- N/A

crates/language/src/proto.rs

@@ -212,7 +212,6 @@ pub fn serialize_diagnostics<'a>(
             } as i32,
             group_id: entry.diagnostic.group_id as u64,
             is_primary: entry.diagnostic.is_primary,
-            is_valid: true,
             code: entry.diagnostic.code.as_ref().map(|s| s.to_string()),
             is_disk_based: entry.diagnostic.is_disk_based,
             is_unnecessary: entry.diagnostic.is_unnecessary,

crates/proto/proto/zed.proto

@@ -2026,8 +2026,7 @@ message Diagnostic {
     uint64 group_id = 7;
     bool is_primary = 8;
 
-    // TODO: remove this field
-    bool is_valid = 9;
+    reserved 9;
 
     bool is_disk_based = 10;
     bool is_unnecessary = 11;