fetch2: Remove broken git variables from the environment

fetch2: Remove broken git variables from the environment

The following variables perform no function outside of bitbake:

    GIT_CONFIG
    GIT_PROXY_HOST
    GIT_PROXY_PORT
    GIT_PROXY_IGNORE

GIT_CONFIG only affects the git-config command which is not relevant to
the fetcher. This was previously used with the OE GIT_CORE_CONFIG
variable which would provide a basic git config to use instead of the
user's config. This usage was deprecated by git for over a year now:

http://git.661346.n2.nabble.com/Overriding-gitconfig-using-GIT-CONFIG-td6680977.html

GIT_PROXY_HOST and GIT_PROXY_PORT are not used by git.

GIT_PROXY_IGNORE was an OE construct used to create the custom git
config and had no meaning outside of the OE environment. It is not used
by git.

Remove these variables from the fetcher environment.

Users wishing to configure git to work with a proxy should define the
GIT_PROXY_COMMAND environment variable to use an external script.
NO_PROXY can be used within this script to skip the proxy for certain
hosts.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>