Makefile: Do not pass MTD_VERSION from the top Makefile

$(MTD_VERSION) is used in tools/env/Makefile

If you specify a variable at a command line like:
  $ make MTD_VERSION=old env
or specify it thru an envrionment variable like:
  $ export MTD_VERSION=old
  $ make env
it is inherited to the sub-make too.
We do not need to pass it from the top Makefile explicitely.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>