Partial fix for building on Mac OS X < 10.12

This commit reverts #3357.

The previous PR is for the new symbol NSEventMaskKeyDown, which is
introduced in #3347. In #3794, #3347 is reverted, so the workaround
in #3357 is no longer needed. Furthermore, it causes build failures
on 10.11 (#3932) as the header file for type NSEventMask is removed
in #3794, too.

Note that this is not a complete fix. A complete patch can be found
at [1]. In MacPorts, the OS version for building a package is the same
as the OS that installs it, so #ifdef can be used to replace @available.
The latter language feature is not available until Xcode 9.

With the patch mentioned in the previous paragraph, KeePassXC 2.5.1
can be built on Mac OS X 10.9 or newer.

Ref: #2899

[1] de1bb703ad/security/KeePassXC/files/patch-old-mac.diff

src/gui/macutils/AppKitImpl.mm

@@ -20,11 +20,6 @@
 
 #import <AppKit/NSWorkspace.h>
 #import <CoreVideo/CVPixelBuffer.h>
-#import <Availability.h>
-
-#if __MAC_OS_X_VERSION_MAX_ALLOWED < 101200
-static const NSEventMask NSEventMaskKeyDown = NSKeyDownMask;
-#endif
 
 @implementation AppKitImpl