ref-manual, yocto-project-qs, poky.ent: Fixed CentOS package

Fixes [YOCTO #12467]

Changed the CentOS variable to use python34-pip
instead of python3-pip. Removed notes indicating
CentOS 6.x exceptions because this version does
not support CentOS 6.x.

(From yocto-docs rev: ad9328b660deced90d13315cff968fb92569eef9)

Signed-off-by: Kristi Rifenbark <kristi@buzzcollectivemarketing.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/poky.ent

@@ -74,5 +74,5 @@
      $ sudo yum makecache
      $ sudo yum install gawk make wget tar bzip2 gzip python unzip perl patch \
      diffutils diffstat git cpp gcc gcc-c++ glibc-devel texinfo chrpath socat \
-     perl-Data-Dumper perl-Text-ParseWords perl-Thread-Queue python3-pip xz \
+     perl-Data-Dumper perl-Text-ParseWords perl-Thread-Queue python34-pip xz \
      which">

documentation/ref-manual/introduction.xml

@@ -351,18 +351,6 @@
             <para>
                 The following list shows the required packages by function
                 given a supported CentOS Linux distribution:
-                <note>
-                    For CentOS 6.x, some of the versions of the components
-                    provided by the distribution are too old (e.g. Git, Python,
-                    and tar).
-                    It is recommended that you install the buildtools in order
-                    to provide versions that will work with the OpenEmbedded
-                    build system.
-                    For information on how to install the buildtools tarball,
-                    see the
-                    "<link linkend='required-git-tar-and-python-versions'>Required Git, Tar, and Python Versions</link>"
-                    section.
-                </note>
                 <itemizedlist>
                     <listitem><para><emphasis>Essentials:</emphasis>
                         Packages needed to build an image for a headless

documentation/yocto-project-qs/yocto-project-qs.xml

@@ -339,15 +339,6 @@
                         </literallayout>
                         <note><title>Notes</title>
                             <itemizedlist>
-                                <listitem><para>
-                                    CentOS 6.x users need to ensure that the
-                                    required versions of Git, tar and Python
-                                    are available.
-                                    For details, See the
-                                    "<ulink url='&YOCTO_DOCS_REF_URL;#required-git-tar-and-python-versions'>Required Git, tar, and Python Versions</ulink>"
-                                    section in the Yocto Project Reference
-                                    Manual for information.
-                                    </para></listitem>
                                 <listitem><para>
                                     Extra Packages for Enterprise Linux
                                     (i.e. <filename>epel-release</filename>)