install-buildtools: remove md5 checksum validation

No need to validate with the md5 checksum, as the file is not even
uploaded to the Yocto release webpage (the download never failed due
to a wrong indentation of an else statement). For validation purposes,
use the sha256 checksum only.

(From OE-Core rev: b331769084996ffeb74007fe6ca7e385edd7a577)

Signed-off-by: Aleksandar Nikolic <aleksandar.nikolic@zeiss.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b740d2f9d40aef1e18c022d1e82b4fb2c5c1fc22)
Signed-off-by: Aleksandar Nikolic <aleksandar.nikolic@zeiss.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>