[gdb/build] Remove superfluous variable param_types in gdb/python/py-param.c

In gdb/python/py-param.c we have:
...
enum param_types
{
  ...
}
param_types;
...
which declares both an enum param_types, and an unused variable param_types.

Fix this by removing the variable.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>