cmd: Remove unreachable code from netboot_nfs()

cmd: Remove unreachable code from netboot_nfs()

rcode is always 0 (CMD_RET_SUCCESS) so last bootstage_error() call is never
executed. Remove it altogether with the rcode local variable.

This supresses following SVACE warning:

* UNREACHABLE_CODE: This statement in the source code might be unreachable during program execution.
     [unreachable] unreachable at cmd/net.c:312
     [rcode != 0 is always false because at this program point the variable rcode is always equal to 0]
      rcode != 0 is always false because at this program point the variable rcode is always equal to 0
      at cmd/net.c:309

Change-Id: Idf4759839897a35fb47870bf953d35bcb5f0cad8
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>