const-time: always avoid comparison operator for byte comparison

* configure.ac: Remove POSSIBLE_CONDITIONAL_BRANCH_IN_BYTE_COMPARISON
macro.
* src/const-time.h (ct_not_equal_byte): Remove
POSSIBLE_CONDITIONAL_BRANCH_IN_BYTE_COMPARISON ifdef.

Performance impact of avoiding comparison is negligible, so
remove the option for using comparison to make this easier
maintain (tested on every arch).

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>