Fix mistaken QPY custom-instructions documentation (Qiskit#12216)

`num_custom_gates` is not actually part of the `CIRCUIT_HEADER` structs
in any version of QPY.  The number of custom-instruction objects is
instead stored as a `uint64_t` inline in the `CUSTOM_DEFINITIONS` part
of the file, so separately to the rest of the header.

qiskit/qpy/__init__.py

@@ -1101,7 +1101,6 @@
         uint64_t metadata_size;
         uint32_t num_registers;
         uint64_t num_instructions;
-        uint64_t num_custom_gates;
     }
 
 This is immediately followed by ``name_size`` bytes of utf8 data for the name
@@ -1134,7 +1133,6 @@
         uint64_t metadata_size;
         uint32_t num_registers;
         uint64_t num_instructions;
-        uint64_t num_custom_gates;
     }
 
 This is immediately followed by ``name_size`` bytes of utf8 data for the name