Why did Thanos remove his armour when getting the Space Stone in Infinity War?

It’s symbolic.

As Thanos gains each Infinity Stone, he begins to lose some kind of outside protection he once received. It’s symbolic of his growing personal power and his decreased reliance on outside aid. He is, in a way, undergoing an apotheosis of sorts.

• After gaining the Space Stone, he sheds his armor. He no longer needs to rely on a suit to protect him from his enemies.
• After gaining the Reality Stone, Ebony Maw is killed thanks to the cleverness of Peter Parker. Thanos loses his most powerful lieutenant, but to quote Maw himself, his powers were “quaint” compared to what the Reality Stone offers.
• He lost the only person he truly loved, Gamora, to gain the Soul Stone.
• He gained the Time Stone without any outside assistance, save the Power, Space, Reality, and Soul Stones. Yes, he lost the rest of the Black Order and the bulk of his Outrider contingent on Earth, but what use was it now? He could control time.
• After completing the Infinity Gauntlet, he probably lost everything he had. Half of his armies would have been decimated, the universe in disarray, and he has nobody to turn to. That’s the price he paid for completing his mission.