lib-index/test-mail-index-transaction-update.c: Cleanup timezone instances

[DimitryAndric]

In 1a7b1f66fe the non-standard extern timezone was partially removed, but some instances were left. Since the test is supposed to run with TZ=UTC, and timezone would have been zero anyway, remove them all.

N.B. I noticed this while compiling dovecot on FreeBSD, where timezone() is actually a function (see https://www.freebsd.org/cgi/man.cgi?query=timezone)! So this resulted in a bunch of pointer arithmetic and integer conversion errors and warnings:

test-mail-index-transaction-update.c:648:42: warning: arithmetic on a pointer to the function type 'char *(int, int)' is a GNU extension [-Wgnu-pointer-arith]
                hdr.day_stamp = tests[i].old_day_stamp + timezone;
                                                       ^ ~~~~~~~~
test-mail-index-transaction-update.c:648:17: error: incompatible pointer to integer conversion assigning to 'uint32_t' (aka 'unsigned int') from 'char *(*)(int, int)' [-Wint-conversion]
                hdr.day_stamp = tests[i].old_day_stamp + timezone;
                              ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test-mail-index-transaction-update.c:650:49: warning: arithmetic on a pointer to the function type 'char *(int, int)' is a GNU extension [-Wgnu-pointer-arith]
                mail_index_update_day_headers(t, tests[i].now + timezone);
                                                              ^ ~~~~~~~~
test-mail-index-transaction-update.c:650:36: error: incompatible pointer to integer conversion passing 'char *(*)(int, int)' to parameter of type 'time_t' (aka 'long') [-Wint-conversion]
                mail_index_update_day_headers(t, tests[i].now + timezone);
                                                 ^~~~~~~~~~~~~~~~~~~~~~~