Make Rust compilation mandatory

The temporary integration check in CheckEquihashSolution() remains, until we
have "real" Rust code to integrate.

Closes #2688.

src/pow.cpp

@@ -16,9 +16,7 @@
 
 #include "sodium.h"
 
-#ifdef ENABLE_RUST
 #include "librustzcash.h"
-#endif // ENABLE_RUST
 
 unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params& params)
 {
@@ -100,13 +98,11 @@ bool CheckEquihashSolution(const CBlockHeader *pblock, const CChainParams& param
     // H(I||V||...
     crypto_generichash_blake2b_update(&state, (unsigned char*)&ss[0], ss.size());
 
-    #ifdef ENABLE_RUST
     // Ensure that our Rust interactions are working in production builds. This is
     // temporary and should be removed.
     {
         assert(librustzcash_xor(0x0f0f0f0f0f0f0f0f, 0x1111111111111111) == 0x1e1e1e1e1e1e1e1e);
     }
-    #endif // ENABLE_RUST
 
     bool isValid;
     EhIsValidSolution(n, k, state, pblock->nSolution, isValid);