Remove obsolete cast

Remove obsolete cast

The upstream timezone code uses a bool variable as an array subscript.
Back when PostgreSQL's bool was char, this would have caused a warning
from gcc -Wchar-subscripts, which is included in -Wall.  But this has
been obsolete since probably commit d26a810ebf9, but certainly since
bool is now the C standard bool.  So we can remove this deviation from
the upstream code, to make future code merges simpler.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/9ad2749f-77ab-4ecb-a321-1ca915480b05%40eisentraut.org