Remove unused argument fLimitFree for AcceptToMemoryPool.

Always enforce limiting of free transactions.
Remove network fee from the code.

It was never used and will never be used, so just clean everything up.
Namecoin is being rebased anyway (without the net fee), so there's not
even a need to keep it to stay in sync with Namecoin.
Merge pull request #88 from domob1812/remove-netfee

Remove "network fee" from code.