qmake: iOS: Don't pass -fobjc-nonfragile-abi -fobjc-legacy-dispatch

According to latest clang source code (llvm-14.0.3) non-fragile ABI
is default for everything but i386 which means we can stop passing the
-fobjc-nonfragile-abi option.

See
https://github.com/llvm/llvm-project/blob/llvmorg-14.0.3/clang/lib/Driver/ToolChains/Darwin.h#L245

This gets rid of the following warning when building a qmake project
for iOS, while targeting the iOS simulator:
  clang: warning: argument unused during compilation:
   '-fobjc-nonfragile-abi' [-Wunused-command-line-argument

It only happens for the simulator case because Xcode adds the
-fobjc-abi-version=2 option, thus the qmake one gets ignored.

As a drive-by also remove the usage of the -fobjc-legacy-dispatch
option. According to clang sources it should be implicitly used
on iOS.
See
https://github.com/llvm/llvm-project/blob/llvmorg-14.0.3/clang/include/clang/Basic/ObjCRuntime.h#L100

Amends d788f636691df5cae59d44a3828a0ddd58c343d0

Pick-to: 6.2 6.3
Fixes: QTBUG-103001
Change-Id: I06f8e46e40a1f04c5bbc4f177afa035e525dfed6
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

mkspecs/common/uikit/clang.conf

@@ -1,7 +1,3 @@
 #
 # compiler settings for iOS/tvOS/watchOS clang compilers
 #
-
-# Based on the following information, http://clang.llvm.org/doxygen/ObjCRuntime_8h_source.html,
-# we can conclude that it's safe to always pass the following flags
-QMAKE_OBJECTIVE_CFLAGS += -fobjc-nonfragile-abi -fobjc-legacy-dispatch