shell: Remove last reference to "INCLUDES"

c61d9e5a95d replaced use of INCLUDES with AM_CPPFLAGS, but there was
still one reference to INCLUDES in shell/Makefile.am.
AM_CPPFLAGS will apply globally, so we don't need to explicitly set
per-target CFLAGS/CPPFLAGS to its value.

https://bugzilla.gnome.org/show_bug.cgi?id=732189