Remove "Not supported yet" message from `machine boot`

[Callum R. Davies]

Based on revisions 1.7 and 1.11, I think this line should have been
removed as part of revision 1.11.

Index: sys/arch/amd64/stand/libsa/cmd_i386.c
===================================================================
RCS file: /cvs/src/sys/arch/amd64/stand/libsa/cmd_i386.c,v
retrieving revision 1.13
diff -u -p -r1.13 cmd_i386.c
--- sys/arch/amd64/stand/libsa/cmd_i386.c       15 Mar 2019 06:53:37 -0000      
1.13
+++ sys/arch/amd64/stand/libsa/cmd_i386.c       6 May 2019 23:43:09 -0000
@@ -96,7 +96,6 @@ Xregs(void)
 int
 Xboot(void)
 {
-       printf("Not supported yet\n");
        int dev, part, st;
        struct diskinfo *dip;
        char buf[DEV_BSIZE], *dest = (void *)BOOTBIOS_ADDR;