chore(r): remove the -D_LIBCPP_DISABLE_AVAILABILITY cpp flag for new macos toolchains (apache#4030)

Fix apache#4025

In the R world, this flag seems no longer needed (added in apache#1672,
apache#3034).

Author: kentkwu

r/adbcdrivermanager/src/Makevars

@@ -17,7 +17,7 @@
 
 CXX_STD = CXX17
 CONDA_BUILD ?= "0"
-PKG_CPPFLAGS=-I../src/c/include -I../src/c -I../src/c/vendor -DADBC_EXPORT="" -D_LIBCPP_DISABLE_AVAILABILITY -DADBC_CONDA_BUILD=$(CONDA_BUILD)
+PKG_CPPFLAGS=-I../src/c/include -I../src/c -I../src/c/vendor -DADBC_EXPORT="" -DADBC_CONDA_BUILD=$(CONDA_BUILD)
 
 OBJECTS = driver_test.o \
     error.o \

r/adbcsqlite/src/Makevars.in

@@ -16,7 +16,7 @@
 # under the License.
 
 CXX_STD = CXX17
-PKG_CPPFLAGS=-I../src/c -I../src/c/include -I../src/c/vendor/ -I../src/c/vendor/fmt/include @cppflags@ -DADBC_EXPORT="" -DFMT_HEADER_ONLY=1 -D_LIBCPP_DISABLE_AVAILABILITY
+PKG_CPPFLAGS=-I../src/c -I../src/c/include -I../src/c/vendor/ -I../src/c/vendor/fmt/include @cppflags@ -DADBC_EXPORT="" -DFMT_HEADER_ONLY=1
 PKG_LIBS=@libs@
 
 OBJECTS = init.o \