MDL-57143 dml: sqlsrv drivers are going xplat \o/

They're now available on PECL:
    https://pecl.php.net/package/sqlsrv

And the source for the extension is on GitHub:
    https://github.com/Microsoft/msphpsql

Note that they do require installation of unixODBC; relevant
documentation is available for Debian- and RHEL-likes:
* https://www.microsoft.com/en-us/sql-server/developer-get-started/php-ubuntu
* https://www.microsoft.com/en-us/sql-server/developer-get-started/php-rhel

lang/en/install.php

@@ -194,7 +194,6 @@ $string['nativesqlsrv'] = 'SQL*Server Microsoft (native/sqlsrv)';
 $string['nativesqlsrvhelp'] = 'Now you need to configure the database where most Moodle data will be stored.
 This database must already have been created and a username and password created to access it. Table prefix is mandatory.';
 $string['nativesqlsrvnodriver'] = 'Microsoft Drivers for SQL Server for PHP are not installed or not configured properly.';
-$string['nativesqlsrvnonwindows'] = 'Microsoft Drivers for SQL Server for PHP are available only for Windows OS.';
 $string['ociextensionisnotpresentinphp'] = 'PHP has not been properly configured with the OCI8 extension so that it can communicate with Oracle.  Please check your php.ini file or recompile PHP.';
 $string['pass'] = 'Pass';
 $string['paths'] = 'Paths';

lib/dml/sqlsrv_native_moodle_database.php

@@ -69,11 +69,7 @@ class sqlsrv_native_moodle_database extends moodle_database {
         // the name used by 'extension_loaded()' is case specific! The extension
         // therefore *could be* mixed case and hence not found.
         if (!function_exists('sqlsrv_num_rows')) {
-            if (stripos(PHP_OS, 'win') === 0) {
-                return get_string('nativesqlsrvnodriver', 'install');
-            } else {
-                return get_string('nativesqlsrvnonwindows', 'install');
-            }
+            return get_string('nativesqlsrvnodriver', 'install');
         }
         return true;
     }