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/Helper/CellValueFormatter.php

@@ -31,14 +31,6 @@ class CellValueFormatter
 
     /** Constants used for date formatting */
     const NUM_SECONDS_IN_ONE_DAY = 86400;
-    const NUM_SECONDS_IN_ONE_HOUR = 3600;
-    const NUM_SECONDS_IN_ONE_MINUTE = 60;
-
-    /**
-     * February 29th, 1900 is NOT a leap year but Excel thinks it is...
-     * @see https://en.wikipedia.org/wiki/Year_1900_problem#Microsoft_Excel
-     */
-    const ERRONEOUS_EXCEL_LEAP_YEAR_DAY = 60;
 
     /** @var SharedStringsManager Manages shared strings */
     protected $sharedStringsManager;