test: Remove UNITTEST params

UNITTEST parameter are not used by any current tests, and the model
(modifyable parameters) is inconvenient when unit-testing. As
they are stored in a global structure eevery test
would have to (re)set up its own parameters.

For consistency it is also better to test with MAIN parameters.

src/chainparamsbase.h

@@ -19,7 +19,6 @@ public:
         MAIN,
         TESTNET,
         REGTEST,
-        UNITTEST,
 
         MAX_NETWORK_TYPES
     };