It is not possible to create a non-incremental backup

Hello,

According to the backup documentation, backups are normally incremental, but it can be configured with the BackupableDBOptions::share_table_files member variable. However, if that variable is set to false, the following assert is thrown:

Assertion failed: share_table_files || !share_files_with_checksum, file C:\DevStudio\extra_x64\rocksdb-7.3.1\include\rocksdb/utilities/backup_engine.h, line 249

... which makes sense, because BackupableDBOptions::share_files_with_checksum member variable is forced to true. As a result, non-incremental backups cannot be created.

I'm currently using version 7.3.1, but I've checked that this code is still there in the last version at the time of writing.

### Expected behavior
To be able to create non-incremental backups.

### Actual behavior
Non-incremental backups cannot be created.

### Steps to reproduce the behavior
Call the ROCKSDB_NAMESPACE::BackupEngine::Open function with BackupEngineOptions::share_table_files option set to true.