refactor: include the proper header rather than forward-declaring RemovalReasonToString #31058

Trivial no-op fixup.

This was pointed out by #31053, which causes the include order to be shuffled around:

[21:49:26.130] /ci_container_base/src/validationinterface.cpp:22:13: error: redundant 'RemovalReasonToString' declaration [readability-redundant-declaration,-warnings-as-errors]
[21:49:26.130]    22 | std::string RemovalReasonToString(const MemPoolRemovalReason& r) noexcept;
[21:49:26.130]       | ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
[21:49:26.130] /ci_container_base/src/kernel/mempool_removal_reason.h:22:13: note: previously declared here
[21:49:26.130]    22 | std::string RemovalReasonToString(const MemPoolRemovalReason& r) noexcept;
[21:49:26.130]       |             ^

I don't see any reason why the include shouldn't just be used.

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lgtm ACK ca2e4ba352cb0a3b5983c002b09ce15ebbf95177

This should be fine, because the circular dependency no longer exists since commit fad8c36aa9011c3f7b1183f8380577e16a2167a6

ACK ca2e4ba352cb0a3b5983c002b09ce15ebbf95177, IWYU seems agree:

[16:19:43.440] The full include-list for /ci_container_base/src/validationinterface.cpp:
[16:19:43.440] #include <validationinterface.h>
[16:19:43.440] #include <chain.h>                          // for CBlockIndex
[16:19:43.440] #include <consensus/validation.h>           // for BlockValidationState
[16:19:43.440] #include <kernel/mempool_entry.h>           // for RemovedMempoolTransactionInfo, NewMempoolTransactionInfo, TransactionInfo
[16:19:43.440] #include <kernel/mempool_removal_reason.h>  // for RemovalReasonToString
[16:19:43.440] #include <logging.h>                        // for LogFlags, LogPrintFormatInternal, LogDebug
[16:19:43.440] #include <primitives/block.h>               // for CBlock, CBlockLocator
[16:19:43.440] #include <primitives/transaction.h>         // for CTransaction, CTransactionRef
[16:19:43.440] #include <util/check.h>                     // for inline_assertion_check, Assert
[16:19:43.440] #include <util/task_runner.h>               // for TaskRunnerInterface
[16:19:43.440] #include <future>                           // for promise, future
[16:19:43.440] #include <iterator>                         // for next
[16:19:43.440] #include <list>                             // for list, _List_iterator, operator==
[16:19:43.440] #include <unordered_map>                    // for unordered_map, _Node_iterator, operator==, _Node_iterator_base
[16:19:43.440] #include <utility>                          // for move, pair
[16:19:43.440] #include "kernel/cs_main.h"                 // for cs_main
[16:19:43.440] #include "sync.h"                           // for AnnotatedMixin, EXCLUSIVE_LOCKS_REQUIRED, UniqueLock, LOCK, GUARDED_BY, AssertLockNotHeld, Mutex, REVERSE...
[16:19:43.440] #include "uint256.h"                        // for uint256
[16:19:43.440] enum class ChainstateRole;
[16:19:43.440] ---

ACK ca2e4ba352cb0a3b5983c002b09ce15ebbf95177