mips: qemu-mips/64: Remove obsolete CONFIG_SYS_MONITOR_LEN from config

This fixes an issue with the saveenv command causing U-Boot to no
longer work on the QEMU Mips pseudoboard. Because the offset of the
environment was being determined by CONFIG_SYS_MONITOR_LEN, and this
value was less than the actual size of U-Boot, saveenv was overwriting
parts of the U-Boot code. Because CONFIG_SYS_MONITOR_LEN is no longer
used on MIPS, this patch removes it and places the environment at the
end of the pseudoboard's 4MB flash.

Signed-off-by: Kyle Edwards <kyleedwardsny@gmail.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>