[Ada] Revert "Global => null" on calendar routines that use timezones

[Pierre-Marie de Rodat]

Some routines from the Ada.Calendar package, i.e. Year, Month, Day,
Split and Time_Off, rely on OS-specific timezone databases that are kept
in files (e.g. /etc/localtime on Linux). In SPARK we want to model this
as a potential side-effect, so those routines can't have "Global =>
null".

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-07-01  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* libgnat/a-calend.ads: Revert "Global => null" contracts on
	non-pure routines.
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