Support for strict OOXML

There are 2 types of OOXML format: transitional and strict. Transitional is what's mostly used but some softwares still allow XLSX to be generated using the strict OOXML format.
In this format, namespaces of the XML files are different: `http://schemas.openxmlformats.org/officeDocument/2006/relationships/sharedStrings` is replaced by `http://purl.oclc.org/ooxml/officeDocument/relationships/sharedStrings` for instance. To support both formats, Spout needs to be able to look for both.

src/Spout/Reader/XLSX/Manager/SharedStringsManager.php

@@ -16,9 +16,6 @@ use Box\Spout\Reader\XLSX\Manager\SharedStringsCaching\CachingStrategyInterface;
  */
 class SharedStringsManager
 {
-    /** Main namespace for the sharedStrings.xml file */
-    const MAIN_NAMESPACE_FOR_SHARED_STRINGS_XML = 'http://schemas.openxmlformats.org/spreadsheetml/2006/main';
-
     /** Definition of XML nodes names used to parse data */
     const XML_NODE_SST = 'sst';
     const XML_NODE_SI = 'si';