Revert "add OPENSSL_X509_CERT_FILE to the `git` command environment"

This reverts commit 88b5578a579ca7eefc8ac5507f9a5be3cc3684ca because of the
following issues:

 1) If $OPENSSL_X509_CERT_FILE is set in the current shell environment, then
    its value will overwrite any settings the user may have configured in
    http.sslCAInfo via git-config(1). If you are unaware of the wrapper, then
    this behavior is totally unexpected as $OPENSSL_X509_CERT_FILE is not
    supposed to have an effect on Git.

 2) The patch makes it impossible for Git users to use the $GIT_SSL_CAINFO
    environment variable as documented in git-config(1), because anything set
    there will be overwritten with the value of $OPENSSL_X509_CERT_FILE (which
    might be empty).

 3) The patch breaks other builds of packages that depend on Git, i.e.
    <http://hydra.nixos.org/build/11995872/nixlog/1/raw>.