Remove useless extern keywords

An extern keyword on a function definition (not declaration) is
useless and not the normal style.

Discussion: https://www.postgresql.org/message-id/flat/e0a62134-83da-4ba4-8cdb-ceb0111c95ce@eisentraut.org

src/backend/backup/basebackup_incremental.c

@@ -944,7 +944,7 @@ GetFileBackupMethod(IncrementalBackupInfo *ib, const char *path,
  * number of blocks. The header is rounded to a multiple of BLCKSZ, but
  * only if the file will store some block data.
  */
-extern size_t
+size_t
 GetIncrementalHeaderSize(unsigned num_blocks_required)
 {
 	size_t		result;
@@ -972,7 +972,7 @@ GetIncrementalHeaderSize(unsigned num_blocks_required)
 /*
  * Compute the size for an incremental file containing a given number of blocks.
  */
-extern size_t
+size_t
 GetIncrementalFileSize(unsigned num_blocks_required)
 {
 	size_t		result;

src/backend/storage/file/fd.c

@@ -4020,7 +4020,7 @@ check_debug_io_direct(char **newval, void **extra, GucSource source)
 	return result;
 }
 
-extern void
+void
 assign_debug_io_direct(const char *newval, void *extra)
 {
 	int		   *flags = (int *) extra;