Remove qtmacextras and qtx11extras submodules

They are no longer part of Qt (as of Qt 6), and the modules themselves
have been wiped (9d99db029613d9cd847407085c5de7f310cd0937, and
6037ece5e4c0d290ae4ebf75c0d9bce0de55ed85).

Task-number: QTBUG-83251
Change-Id: Ie3acfc6d31bda0fcc1c6189e08171dbdf3c50ff7
Reviewed-by: Liang Qi <liang.qi@qt.io>

.gitmodules

@@ -158,18 +158,6 @@
 	url = ../qtserialport.git
 	branch = dev
 	status = addon
-[submodule "qtx11extras"]
-	depends = qtbase
-	path = qtx11extras
-	url = ../qtx11extras.git
-	branch = dev
-	status = ignore
-[submodule "qtmacextras"]
-	depends = qtbase
-	path = qtmacextras
-	url = ../qtmacextras.git
-	branch = dev
-	status = ignore
 [submodule "qtwinextras"]
 	depends = qtbase
 	recommends = qtdeclarative qtmultimedia