Do not include py-ref.h in most files

py-ref.h can really only be included from a specific spot in
python-internal.h.  The other includes are not useful, and cause
compilation errors if the includes are ever sorted.  So, remove these
includes.

Arguably, py-ref.h should simply not be a separate header.

gdb/ChangeLog
2019-01-22  Tom Tromey  <tom@tromey.com>

	* python/py-arch.c: Do not include py-ref.h.
	* python/py-bpevent.c: Do not include py-ref.h.
	* python/py-cmd.c: Do not include py-ref.h.
	* python/py-continueevent.c: Do not include py-ref.h.
	* python/py-event.h: Do not include py-ref.h.
	* python/py-evtregistry.c: Do not include py-ref.h.
	* python/py-finishbreakpoint.c: Do not include py-ref.h.
	* python/py-frame.c: Do not include py-ref.h.
	* python/py-framefilter.c: Do not include py-ref.h.
	* python/py-function.c: Do not include py-ref.h.
	* python/py-infevents.c: Do not include py-ref.h.
	* python/py-linetable.c: Do not include py-ref.h.
	* python/py-objfile.c: Do not include py-ref.h.
	* python/py-param.c: Do not include py-ref.h.
	* python/py-prettyprint.c: Do not include py-ref.h.
	* python/py-progspace.c: Do not include py-ref.h.
	* python/py-symbol.c: Do not include py-ref.h.
	* python/py-symtab.c: Do not include py-ref.h.
	* python/py-type.c: Do not include py-ref.h.
	* python/py-unwind.c: Do not include py-ref.h.
	* python/py-utils.c: Do not include py-ref.h.
	* python/py-value.c: Do not include py-ref.h.
	* python/py-varobj.c: Do not include py-ref.h.
	* python/py-xmethods.c: Do not include py-ref.h.
	* python/python.c: Do not include py-ref.h.
	* varobj.c: Do not include py-ref.h.