Remove PropertyMap from XMLNode class

Remove PropertyMap from XMLNode class

It appears that there is no performance benefit from storing properties in a
map for faster lookup or it is counteracted by the penalty of storing and
maintaining the additional data structure.

Timing results before changes with an optimized build:

XMLTest::testPerfMediumXMLDocumentTiming
   Create : Count: 10 Min: 41293 Max: 63746 Total: 564448 Avg: 56444 (56 msecs)
   Write : Count: 10 Min: 42932 Max: 49221 Total: 453955 Avg: 45395 (45 msecs)
   Read : Count: 10 Min: 80160 Max: 84678 Total: 824506 Avg: 82450 (82 msecs)

XMLTest::testPerfLargeXMLDocumentTiming
   Create : Count: 10 Min: 228759 Max: 420236 Total: 3587597 Avg: 358759 (358 msecs)
   Write : Count: 10 Min: 307095 Max: 348767 Total: 3205704 Avg: 320570 (320 msecs)
   Read : Count: 10 Min: 572400 Max: 657219 Total: 5959630 Avg: 595963 (595 msecs)

Perf results after changes:

XMLTest::testPerfMediumXMLDocumentTiming
   Create : Count: 10 Min: 30610 Max: 42656 Total: 376672 Avg: 37667 (37 msecs)
   Write : Count: 10 Min: 42804 Max: 54277 Total: 460455 Avg: 46045 (46 msecs)
   Read : Count: 10 Min: 70364 Max: 85484 Total: 750909 Avg: 75090 (75 msecs)

XMLTest::testPerfLargeXMLDocumentTiming
   Create : Count: 10 Min: 164360 Max: 356995 Total: 3064482 Avg: 306448 (306 msecs)
   Write : Count: 10 Min: 308655 Max: 372953 Total: 3226707 Avg: 322670 (322 msecs)
   Read : Count: 10 Min: 517243 Max: 541839 Total: 5289950 Avg: 528995 (528 msecs)