gh-64502: Convert curses methods with optional groups to Argument Clinic

chgat(), getstr(), instr(), get_wstr(), in_wstr() and in_wchstr() were
parsing their arguments by hand, because Argument Clinic did not support
their signatures.  Their helper for parsing the optional y, x and n
arguments is no longer needed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: serhiy-storchaka