Remove unused function prototypes.

Cleanup for commit dee663f7.
Reported-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Discussion: https://postgr.es/m/CA+hUKGLJ=84YT+NvhkEEDAuUtVHMfQ9i-N7k_o50JmQ6Rpj_OQ@mail.gmail.com

src/include/access/clog.h

@@ -46,7 +46,6 @@ extern void CLOGShmemInit(void);
 extern void BootStrapCLOG(void);
 extern void StartupCLOG(void);
 extern void TrimCLOG(void);
-extern void ShutdownCLOG(void);
 extern void CheckPointCLOG(void);
 extern void ExtendCLOG(TransactionId newestXact);
 extern void TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid);

src/include/access/commit_ts.h

@@ -38,7 +38,6 @@ extern void BootStrapCommitTs(void);
 extern void StartupCommitTs(void);
 extern void CommitTsParameterChange(bool newvalue, bool oldvalue);
 extern void CompleteCommitTsInitialization(void);
-extern void ShutdownCommitTs(void);
 extern void CheckPointCommitTs(void);
 extern void ExtendCommitTs(TransactionId newestXact);
 extern void TruncateCommitTs(TransactionId oldestXact);

src/include/access/multixact.h

@@ -130,7 +130,6 @@ extern void MultiXactShmemInit(void);
 extern void BootStrapMultiXact(void);
 extern void StartupMultiXact(void);
 extern void TrimMultiXact(void);
-extern void ShutdownMultiXact(void);
 extern void SetMultiXactIdLimit(MultiXactId oldest_datminmxid,
 								Oid oldest_datoid,
 								bool is_startup);

src/include/access/subtrans.h

@@ -22,7 +22,6 @@ extern Size SUBTRANSShmemSize(void);
 extern void SUBTRANSShmemInit(void);
 extern void BootStrapSUBTRANS(void);
 extern void StartupSUBTRANS(TransactionId oldestActiveXID);
-extern void ShutdownSUBTRANS(void);
 extern void CheckPointSUBTRANS(void);
 extern void ExtendSUBTRANS(TransactionId newestXact);
 extern void TruncateSUBTRANS(TransactionId oldestXact);