Remove obsolete info regarding libgd < 2.1.0

As of PHP 7.0.0, libgd ≥ 2.1.0 is required anyway.

reference/image/book.xml

@@ -119,7 +119,7 @@
        <entry>BMP</entry>
        <entry>&true;</entry>
        <entry>&true;</entry>
-       <entry>Available as of GD 2.1.0 and PHP 7.2.0</entry>
+       <entry>Available as of PHP 7.2.0</entry>
       </row>
      </tbody>
     </tgroup>

reference/image/functions/imageconvolution.xml

@@ -131,12 +131,6 @@ imagepng($image, null, 9);
    </example>
   </para>
  </refsect1>
- <refsect1 role="notes">
-  &reftitle.notes;
-  <para>
-   This function requires GD 2.1.0 or higher.
-  </para>
- </refsect1>
  <refsect1 role="seealso">
   &reftitle.seealso;
   <simplelist>

reference/image/functions/imagecreatefromgif.xml

@@ -114,16 +114,6 @@ imagedestroy($img);
    </mediaobject>
   </example>
  </refsect1>
- <refsect1 role="notes">
-  &reftitle.notes;
-  <note>
-   <para>
-    GIF support was removed from the GD library in Version 1.6,
-    and added back in Version 2.0.28. This function is not
-    available between these versions.
-   </para>
-  </note>
- </refsect1>
 </refentry>
 <!-- Keep this comment at the end of the file
 Local variables:

reference/image/functions/imagefttext.xml

@@ -96,13 +96,6 @@
        to the filename and the library will attempt to search for that
        filename along a library-defined font path.
       </para>
-      <para>
-       When using versions of the GD library lower than 2.0.18, a <literal>space</literal> character,
-       rather than a semicolon, was used as the 'path separator' for different font files.
-       Unintentional use of this feature will result in the warning message:
-       <literal>Warning: Could not find/open font</literal>. For these affected versions, the
-       only solution is moving the font to a path which does not contain spaces.
-      </para>
       <para>
        In many cases where a font resides in the same directory as the script using it
        the following trick will alleviate any include problems.

reference/image/functions/imagegif.xml

@@ -117,12 +117,6 @@ echo 'Converted PNG image to GIF with success!';
  <refsect1 role="notes">
   &reftitle.notes;
   <note>
-   <para>
-    <acronym>GIF</acronym> support was removed from the <acronym>GD</acronym>
-    library in Version 1.6, and added back in Version 2.0.28. This function 
-    is not available between these versions.  For more information, see the
-    <link xlink:href="&url.gd;">GD Project</link> site.
-   </para>      
    <para>
     The following code snippet allows you to write more
     portable PHP applications by auto-detecting the