refactor: include the proper header rather than forward-declaring RemovalReasonToString

This was not in its own header when it was added, but now that it is the
forward-declare makes no sense.

src/validationinterface.cpp

@@ -9,6 +9,7 @@
 #include <consensus/validation.h>
 #include <kernel/chain.h>
 #include <kernel/mempool_entry.h>
+#include <kernel/mempool_removal_reason.h>
 #include <logging.h>
 #include <primitives/block.h>
 #include <primitives/transaction.h>
@@ -19,8 +20,6 @@
 #include <unordered_map>
 #include <utility>
 
-std::string RemovalReasonToString(const MemPoolRemovalReason& r) noexcept;
-
 /**
  * ValidationSignalsImpl manages a list of shared_ptr<CValidationInterface> callbacks.
  *