Remove Android's forked libselinux

libselinux is now built from the external selinux project in
external/selinux/libselinux. Remove the Android fork located in
external/libselinux.
Test: Build and boot Hikey
Bug: 32376472
Bug: 31910164
Change-Id: I0dac8275f7d0fdc50f8609fc5c4e0b4fae7da638

default.xml

@@ -210,7 +210,6 @@
   <project path="external/libpcap" name="platform/external/libpcap" groups="pdk" />
   <project path="external/libphonenumber" name="platform/external/libphonenumber" groups="pdk" />
   <project path="external/libpng" name="platform/external/libpng" groups="pdk" />
-  <project path="external/libselinux" name="platform/external/libselinux" groups="pdk" />
   <project path="external/libunwind" name="platform/external/libunwind" groups="pdk" />
   <project path="external/libunwind_llvm" name="platform/external/libunwind_llvm" groups="pdk" />
   <project path="external/libusb" name="platform/external/libusb" groups="pdk" />