Drop macOS ForceActivation workaround

[fanquake]

Discussion in bitcoin/bitcoin#16720 seems to point to this being no-longer needed after qt 5.6+.
We now require 5.11.x+.

[DrahtBot]

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Concept ACK	hebasto

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[hebasto]

Concept ACK.

We now require 5.11.x+.

When building on macOS, Homebrew provides Qt 5.15.10. Our depends does 5.15.5.

cc @Sjors

[hebasto]

I think we can drop -DOBJC_OLD_DISPATCH_PROTOTYPES=0 now. At least, the code compiles for me natively on macOS Ventura (arm64) and when cross-building on Ubuntu 23.04.

[Sjors]

Tested 362e989 but it seems the workaround is still needed.

This was introduced in bitcoin/bitcoin#14123 as a fix to bitcoin/bitcoin#13829.

With this PR, built on macOS 13.4.1 (Intel) with qt@5/5.15.10 via Homebrew, when I use ⌘ + H to hide the window, and then e.g. click on "Receive" in the dock, the window doesn't appear. Without this PR the window does appear.