ui/cocoa.m: Remove unnecessary NSAutoreleasePools

In commit 6e657e64 in 2013 we added some autorelease pools to
deal with complaints from macOS when we made calls into Cocoa from
threads that didn't have automatically created autorelease pools.
Later on, macOS got stricter about forbidding cross-thread Cocoa
calls, and in commit 5588840f we restructured the code to
avoid them.  This left the autorelease pool creation in several
functions without any purpose; delete it.

We still need the pool in cocoa_refresh() for the clipboard related
code which is called directly there.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Tested-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Message-id: 20220224101330.967429-3-peter.maydell@linaro.org