Remove unused project external/libppp.

Bug: 16795898

default.xml

@@ -176,7 +176,6 @@
   <project path="external/libpcap" name="platform/external/libpcap" />
   <project path="external/libphonenumber" name="platform/external/libphonenumber" />
   <project path="external/libpng" name="platform/external/libpng" groups="pdk" />
-  <project path="external/libppp" name="platform/external/libppp" />
   <project path="external/libseccomp-helper" name="platform/external/libseccomp-helper" />
   <project path="external/libselinux" name="platform/external/libselinux" groups="pdk" />
   <project path="external/libsepol" name="platform/external/libsepol" groups="pdk" />