The patch refactors a function `datetime_parse_full()` -
overriding of timezone is not a part of datetime string parsing
and this part was removed.

Needed for #8333

NO_CHANGELOG=refactoring
NO_DOC=refactoring
NO_TEST=refactoring

(cherry picked from commit d7d3063fbd5a74563fde539f2c74852a1e04c1cd)