remove external/libnl-headers from the manifest

external/libnl-headers was part of system/core/libnl_2, which has
been replaced by external/libnl
Change-Id: I19b3bf9770ea5684bccee512686322d9f5dc033a

default.xml

@@ -154,7 +154,6 @@
   <project path="external/libnfc-nci" name="platform/external/libnfc-nci" groups="pdk" />
   <project path="external/libnfc-nxp" name="platform/external/libnfc-nxp" groups="pdk" />
   <project path="external/libnl" name="platform/external/libnl" />
-  <project path="external/libnl-headers" name="platform/external/libnl-headers" groups="pdk" />
   <project path="external/libogg" name="platform/external/libogg" />
   <project path="external/libpcap" name="platform/external/libpcap" />
   <project path="external/libphonenumber" name="platform/external/libphonenumber" />