Strip data-parsoid.src from tpls/extension nodes when unnecessary

* Since templates and extensions in the regular serializer are
  serialized from data-mw, dp.src is never used except when data-mw
  is missing (which it should not be if clients are doing their
  job correctly).  This should reduce HTML size a bit.

* Reduces HTML size for the BO page by 13%.

* An unclosed-comment ref test is now passing wt2html mode because
  the unclosed comment is no longer in the output because of dp.src
  stripping.  Earlier, the unclosed comment in dp.src also tripped
  up our normalization code and artificially caused a test failure.

Change-Id: I788b3099d306d0fe4d368af42a90999c7f21ace1