**/Makefile.am: remove unused AM_CPPFLAGS

only 1 of <lib>_CPPFLAGS and AM_CPPFLAGS is used, with the former
getting precedence when it's defined. configure's DEFAULT_INCLUDES is
covering what's necessary given the include paths are all source
relative.

Change-Id: I7d14076acd266b28a88a3d92bcc3d7165284d5f3