ffi: drop legacy-variadic-form migration diagnostic

The special-case `return self.fail("legacy variadic syntax ...")`
in `parseParams` is gone. `parseTypeExpr` already errors naturally
on a leading `..` (now reported as "expected type name"), which
is enough — the language-level cutover happened in the previous
commit; no need for the parser to keep a migration breadcrumb.

src/parser.zig

@@ -1631,16 +1631,6 @@ pub const Parser = struct {
                 continue;
             }
             self.advance(); // consume ':'
-            // The legacy `name: ..T` form is rejected — write `..name: []T`.
-            // Both forms used to live; the leading-`..` form is the one the
-            // language settled on (the AST shape downstream assumes the
-            // declared type IS the slice). Leaving the post-colon form
-            // accepted lets users land code that bypasses the new-form
-            // lowering paths (resolveParamType / packVariadicCallArgs) and
-            // hit cross-module emit crashes (issue-0049). Hard error here.
-            if (self.current.tag == .dot_dot) {
-                return self.fail("legacy variadic syntax `name: ..T` is no longer supported — use `..name: []T` instead");
-            }
             const param_type = try self.parseTypeExpr();
             var is_comptime_param = false;
             if (is_ct_param and param_type.data == .type_expr) {