Filter U gate from output of SymbolTable::gates

The Qiskit importer otherwise needs to filter this gate out.
Other consumers probably would do so as well. If it is useful in
antother context, we can add an option to include the U gate

Review comment: Qiskit/qiskit#12087 (comment)

crates/oq3_semantics/src/symbols.rs

@@ -251,13 +251,19 @@ impl SymbolTable {
 
     /// Return a Vec of information about all gate declarations. Each element
     /// is a tuple of (gate name, symbol id, num classical params, num quantum params).
+    /// `gphase` is not included here. It is treated specially.
+    /// `U` is also filtered out, as it is builtin.
     pub fn gates(&self) -> Vec<(&str, SymbolId, usize, usize)> {
         self.all_symbols
             .iter()
             .enumerate()
             .filter_map(|(n, sym)| {
                 if let Type::Gate(num_cl, num_qu) = &sym.symbol_type() {
-                    Some((sym.name(), SymbolId(n), *num_cl, *num_qu))
+                    if sym.name() == "U" {
+                        None
+                    } else {
+                        Some((sym.name(), SymbolId(n), *num_cl, *num_qu))
+                    }
                 } else {
                     None
                 }