Cleanup: Remove unused error code DB_FORCED_ABORT

MariaDB never supported this form of preemption via high-priority
transactions. This error code shold not have been added in the
first place, in commit 2e814d47.

storage/innobase/include/db0err.h

 /*****************************************************************************
 
 Copyright (c) 1996, 2016, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2015, 2018, MariaDB Corporation.
+Copyright (c) 2015, 2022, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -148,9 +148,6 @@ enum dberr_t {
 
 	DB_IO_PARTIAL_FAILED,		/*!< Partial IO request failed */
 
-	DB_FORCED_ABORT,		/*!< Transaction was forced to rollback
-					by a higher priority transaction */
-
 	DB_TABLE_CORRUPT,		/*!< Table/clustered index is
 					corrupted */
 

storage/innobase/ut/ut0ut.cc

 /*****************************************************************************
 
 Copyright (c) 1994, 2017, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2017, 2021, MariaDB Corporation.
+Copyright (c) 2017, 2022, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -486,9 +486,6 @@ ut_strerr(
 		return("Table is encrypted but decrypt failed.");
 	case DB_IO_PARTIAL_FAILED:
 		return("Partial IO failed");
-	case DB_FORCED_ABORT:
-		return("Transaction aborted by another higher priority "
-		       "transaction");
 	case DB_COMPUTE_VALUE_FAILED:
 		return("Compute generated column failed");
 	case DB_NO_FK_ON_S_BASE_COL: