gnu: optipng: Update to 0.7.7 [security fixes].

gnu: optipng: Update to 0.7.7 [security fixes].

This release claims to fix 2 vulnerabilities:
- ‘an integer overflow vulnerability in the TIFF decoder’
  (CVE-2017-1000229, previously patched in Guix), and
- ‘a buffer overflow vulnerability in the GIF decoder’.

* gnu/packages/image.scm (optipng): Update to 0.7.7.
[source]: Remove patch.
[arguments]: Substitute INVOKE for SYSTEM* and end phase with #t.
* gnu/packages/patches/optipng-CVE-2017-1000229.patch: Delete file.
* gnu/local.mk (dist_patch_DATA): Remove it.