Update the googletest version setting in .gitmodules

Author: tuhaihe

.gitmodules

@@ -4,7 +4,6 @@
 [submodule "contrib/pax_storage/src/cpp/contrib/googletest"]
 	path = contrib/pax_storage/src/cpp/contrib/googletest
 	url = https://github.com/google/googletest.git
-	branch = v1.15.x
 [submodule "contrib/pax_storage/src/cpp/contrib/tabulate"]
 	path = contrib/pax_storage/src/cpp/contrib/tabulate
 	url = https://github.com/p-ranav/tabulate.git