libecal: importing event with pseudo "Etc/UTC" timezone crashed (#593019)

The problem was two-fold: first, e_cal_match_timezone() recognized the
pseudo VTIMEZONE as matching the internal "UTC". Then e_cal_get_timezone()
failed to extract this internal VTIMEZONE because there is no real
definition for it, leading to an assert which terminates the caller.

Matching to "UTC" is also problematic because e_cal_match_timezones()
will use it for the imported event, which then cannot be exported
properly (leads to TZID=UTC without VTIMEZONE definition).

Therefore this patch removes the mapping to "UTC" in e_cal_match_timezone().
It also fixes e_cal_get_timezone() so that it does not assert and
returns an error code, as originally intented, although that should
no longer happen with the change to e_cal_match_timezone().