docs: remove pkg-config from requirements

This was removed on 2014-10-29[1], but the docs weren't updated.

  - cross packages use the prefixed ${target}-pkg-config wrapper
  - native packages use usr/${build}/bin/pkgconf
  - native reqs may install whichever pkg-config the package manager
    specifies, but it isn't used by mxe

closes #2456

[1] a063f823e8 (diff-b67911656ef5d18c4ae36cb6741b7965)

docs/index.html

@@ -670,10 +670,6 @@ USE_OSGPLUGIN(<plugin2>)
         <td><a href="https://metacpan.org/pod/distribution/XML-Parser/Parser.pm">Perl XML::Parser</a></td>
         <td></td>
     </tr>
-    <tr>
-        <td><a href="https://www.freedesktop.org/wiki/Software/pkg-config/">Pkg-config</a></td>
-        <td>≥ 0.16</td>
-    </tr>
     <tr>
         <td><a href="https://www.python.org/">Python</a></td>
         <td></td>
@@ -729,7 +725,6 @@ USE_OSGPLUGIN(&lt;plugin2&gt;)
     p7zip-full \
     patch \
     perl \
-    pkg-config \
     python \
     ruby \
     sed \
@@ -787,7 +782,6 @@ USE_OSGPLUGIN(&lt;plugin2&gt;)
     p7zip \
     patch \
     perl \
-    pkgconfig \
     python \
     ruby \
     sed \
@@ -825,7 +819,6 @@ USE_OSGPLUGIN(&lt;plugin2&gt;)
     p7zip \
     patch \
     perl5 \
-    pkgconf \
     python \
     ruby \
     unzip \
@@ -895,7 +888,6 @@ USE_OSGPLUGIN(&lt;plugin2&gt;)
     patch \
     perl \
     perl-xml-parser \
-    pkgconfig \
     python \
     ruby \
     sed \
@@ -926,7 +918,6 @@ USE_OSGPLUGIN(&lt;plugin2&gt;)
     dev-perl/XML-Parser \
     dev-util/gperf \
     dev-util/intltool \
-    dev-util/pkgconfig \
     dev-vcs/git \
     net-misc/wget \
     sys-apps/sed \
@@ -988,7 +979,6 @@ USE_OSGPLUGIN(&lt;plugin2&gt;)
     libtool \
     lzip \
     p7zip \
-    pkg-config \
     wget \
     xz</pre>
     <p>
@@ -1042,7 +1032,6 @@ USE_OSGPLUGIN(&lt;plugin2&gt;)
     patch \
     perl \
     perl-XML-Parser \
-    pkg-config \
     python \
     ruby \
     sed \