util: Unset LC_* variables as they may be unset at login time

Otherwise, a switch from a mixed locale where LANG != REGION(/FORMAT) to a
non-mixed environment where LANG == REGION(/FORMAT) may result in LC_*
surviving in systemd and the change not getting applied.

Similar issues might occur when other DEs are setting these variables
before a login into GNOME happens.

Closes: #85