Remove fromhex implementation from gdbreplay

gdbreplay had its own implementation of fromhex.  This patch changes
it to use the one in common/.

gdb/gdbserver/ChangeLog
2019-06-02  Tom Tromey  <tom@tromey.com>

	* gdbreplay.c (fromhex): Remove.
	* Makefile.in (GDBREPLAY_OBS): Add rsp-low.o.