gh-126004: Remove redundant safeguards for codecs handlers tests (#127680)

We remove the safeguards that were added in `Lib/test/test_capi/test_codecs.py`
since they are now redundant (see 32e07fd377f81cbeb8c108fc791a3e7d631319b6
for additional context).

Indeed, the codecs handlers now correctly handle the `start` and `end` positions
of `UnicodeError` objects and thus should not crash.