Don't include readline headers from tracepoint.c

Don't include readline headers from tracepoint.c

I was curious why updating readline caused so much to be to rebuilt,
so I look at all the uses of the readline headers.  Most are included
for valid reasons (either readline directory or for tilde_expand); but
the includes in tracepoint.c didn't seem to be used.

This patch removes them.  Tested by rebuilding.

gdb/ChangeLog
2019-08-13  Tom Tromey  <tromey@adacore.com>

	* tracepoint.c: Don't include readline.h or history.h.