Revert "Document that Python 2.3 requires cdecimal module for full functionality."

This reverts commit 2a33b79.
The change requiring cdecimal is new in 9.4 (see 7919398),
so we should not claim previous branches need it.

Author: tglsfdc

doc/src/sgml/installation.sgml

@@ -206,12 +206,8 @@ su - postgres
       language, you need a <productname>Python</productname>
       installation with the header files and
       the <application>distutils</application> module.  The minimum
-      required version is <productname>Python</productname> 2.3.
-      For full functionality, a 2.3.x installation must include the
-      separately-available <filename>cdecimal</> module; in particular
-      the <application>PL/Python</> regression tests will not pass if that
-      is missing.
-      <productname>Python 3</productname> is supported if it's
+      required version is <productname>Python</productname>
+      2.3.  <productname>Python 3</productname> is supported if it's
       version 3.1 or later; but see
       <![%standalone-include[the <application>PL/Python</> documentation]]>
       <![%standalone-ignore[<xref linkend="plpython-python23">]]>