Includes fixes for CVE-2018-18500, CVE-2018-18501, and CVE-2018-18505.
* gnu/packages/gnuzilla.scm (icecat): Update to 60.5.0-guix1.
[version]: Use %icecat-version.
[source]: Inherit from 'icecat-source'. Remove obsolete patches.
* gnu/packages/patches/icecat-avoid-bundled-libraries.patch,
gnu/packages/patches/icecat-use-system-graphite2+harfbuzz.patch,
gnu/packages/patches/icecat-use-system-media-libs.patch: Adapt to 60.5.0.