gh-97588: remove unused functions in _ctypes/cfield.c

[iritkatriel]

./Modules/_ctypes/cfield.c:49:12: warning: unused function 'round_down' [-Wunused-function]
Py_ssize_t round_down(Py_ssize_t numToRound, Py_ssize_t multiple)
           ^
./Modules/_ctypes/cfield.c:59:12: warning: unused function 'round_up' [-Wunused-function]
Py_ssize_t round_up(Py_ssize_t numToRound, Py_ssize_t multiple)
           ^
./Modules/_ctypes/cfield.c:409:12: warning: unused function 'BUILD_SIZE' [-Wunused-function]
Py_ssize_t BUILD_SIZE(Py_ssize_t bitsize, Py_ssize_t offset) {

• Issue: ctypes: bit field data does not survive round trip #97588

[encukou]

Thanks for the catch!

[vstinner]

Post-merge review: LGTM :-)