Use req.get_param's store feature in lieu of utils.purge

This patch removes the use of utils.purge since it is no
longer needed in light of Falcon's relatively new "store"
feature, which causes the target dict to only receive the
value of the param if it is found in the request, thus
avoiding clobbering default kwarg values, which was the
original intent of utils.purge.

As a bonus, this approach is also ~40% faster.

Change-Id: I6b3d82b1f700045e50b3131902b2c7b6dc6ea13c
Implements: blueprint v1-obvious-optimizations
Merge "Use req.get_param's store feature in lieu of utils.purge"