HPCC-10195 Remove unused m_allowHttpNil flag from CSoapPrimitiveField

These classes are used by soapclient.cpp which doesn't need logic
dealing with HTTP URL and Form parameters.

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
Merge pull request #4986 from afishbeck/http_nil

HPCC-10195 Remove unused m_allowHttpNil flag from CSoapPrimitiveField

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>