serial-forward: fix fetch error

removed protcol parameter.

WARNING: Failed to fetch URL git://github.com/freesmartphone/cornucopia.git;protocol=https

, attempting MIRRORS if available
ERROR: Fetcher failure: Fetch command failed with exit code 128, output:
Cloning into bare repository '/home/akuster/oss/downloads/git2/github.com.freesmartphone.cornucopia.git'...
fatal: Unable to find remote helper for 'https'

Signed-off-by: Armin Kuster <akuster808@gmail.com>