Fixing make step on *nix

beam.h was removed here
https://github.com/JohnLangford/vowpal_wabbit/commit/b01f6b25d797b60f19dbbc2d879c158c01974216

Removing it from Makefile.am to fix make step, otherwise make finishes with error status:

make[1]: Leaving directory `/srv/rtb/tmp/vowpal_wabbit/library'
make[1]: Entering directory `/srv/rtb/tmp/vowpal_wabbit'
make[1]: *** No rule to make target `vowpalwabbit/beam.h', needed by `all-am'.  Stop.
make[1]: Leaving directory `/srv/rtb/tmp/vowpal_wabbit'
make: *** [all-recursive] Error 1