[pxe] Remove pxe_set_cached_filename()

gPXE currently overwrites the filename stored in the cached DHCP
packets when a call to PXENV_TFTP_READ_FILE or PXENV_RESTART_TFTP is
made.  This code has existed for many years as a workaround for RIS,
which seemed to require that this be done.

pxe_set_cached_filename() causes problems with the Bootix NBP, and a
recent test demonstrates that RIS will complete successfully even with
pxe_set_cached_filename() removed.  There have been many changes to
the DHCP and PXE logic since this code was first added, and it is
quite plausible that it was masking a bug that no longer exists.

Reported-by: Alex Zeffertt <alex.zeffertt@eu.citrix.com>
Debugged-by: Shao Miller <Shao.Miller@yrdsb.edu.on.ca>
Signed-off-by: Michael Brown <mcb30@etherboot.org>

src/arch/i386/include/pxe.h

@@ -149,6 +149,4 @@ extern struct net_device *pxe_netdev;
 
 extern void pxe_set_netdev ( struct net_device *netdev );
 
-extern void pxe_set_cached_filename ( const unsigned char *filename );
-
 #endif /* PXE_H */