graphviz: drop unused SRCREV and outdated comment

* the SRCREV isn't used as we don't use git fetcher here and comment
  doesn't make sense anymore as we're fetching versioned tarball now after:

  commit 985be3901e
  Author: Khem Raj <raj.khem@gmail.com>
  Date:   Mon Nov 2 18:33:34 2020 -0800

    graphviz: Upgrade to 2.44.1 release

    - Refresh patches to apply on new sources
    - Switch away from gitlab archives
    - Bypass pdf documentation generation

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-oe/recipes-graphics/graphviz/graphviz_2.44.1.bb

@@ -20,13 +20,6 @@ DEPENDS_append_class-nativesdk = " ${BPN}-native"
 
 inherit autotools-brokensep pkgconfig gettext
 
-# The source tarball suggested at
-# https://graphviz.gitlab.io/_pages/Download/Download_source.html has no
-# version in its name. So once graphviz is updgraded, only first time users will
-# get checksum errors. Fedora people seem to expect same so they use a versioned
-# source - see https://src.fedoraproject.org/cgit/rpms/graphviz.git/tree/graphviz.spec
-
-SRCREV = "771bc4dbff3e6f358fa75cdc7774a413ccacad51"
 SRC_URI = "https://www2.graphviz.org/Packages/stable/portable_source/${BP}.tar.gz \
            file://0001-plugin-pango-Include-freetype-headers-explicitly.patch \
 "