Remove outdated feature gate from proc macro test

Author: lnicola

crates/rust-analyzer/tests/slow-tests/main.rs

@@ -832,7 +832,6 @@ fn main() {
 }
 
 #[test]
-#[cfg(feature = "sysroot-abi")]
 fn resolve_proc_macro() {
     use expect_test::expect;
     if skip_slow_tests() {