git: remove symling before updating mirror tarball

* with read-only PREMIRROR (e.g. mounted over NFS or CIFS
  and referenced as file:///mnt/premirror) we cannot use
  BB_GENERATE_MIRROR_TARBALLS because all git2_abc.git.tar.gz
  files later became just symlinks to read-only location in PREMIRROR
  (it works fine on first build and for new components, because
  at that time there isn't tarball on PREMIRROR yet).

  ERROR: Fetcher failure: Fetch command failed with exit code 141, output:
  tar (child): /build/downloads/git2_abc.git.tar.gz: Cannot open: Read-only file system
  tar (child): Error is not recoverable: exiting now

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>